Add UI shortcuts for document/file rename feature

Accessible from MainMenu->Document->Rename, Documents sidebar tab context menu and tab label context menu. It opens the Save As dialog which will be in the document's directory and have the document's basename up to the extension selected by default. The Save As dialog's Rename button is set to be the default response (ie. highlighted by default and activated when Enter is pressed).

If the document is not saved on disk yet, the opened Save As dialog will have the Save button set as the dialog's default response (as usual).

#### Use case

I often want to rename files in the same directory quickly, ex. changing "stuff.c" to "myns-stuff.c" or changing C++ headers from "foo.h" to "foo.hpp" or simple things like this and minimizing the steps to perform a rename by having the Rename button as the default response is useful and (IMO) the new menu items are slightly more intuitive than the button in the bottom of Save As dialog alone.

#### Related feature requests

I just added these for general interest, they have already been 95% addressed by the previous addition of the Rename feature in the Save As dialog.

http://sourceforge.net/tracker/index.php?func=detail&aid=3027545&group_id=153444&atid=787794
http://sourceforge.net/tracker/index.php?func=detail&aid=1702822&group_id=153444&atid=787794
http://sourceforge.net/tracker/index.php?func=detail&aid=1729010&group_id=153444&atid=787794

#### Screenshots

![The Save As dialog with the Rename as the default response](https://f.cloud.github.com/assets/181177/491983/8a02f95e-ba72-11e2-86f2-62b5bd0a988a.png)

![The item in the main Document menu](https://f.cloud.github.com/assets/181177/486674/9afcfce8-b930-11e2-9404-b4677023c343.png)

![The item in the context menu of the sidebar's Documents tab](https://f.cloud.github.com/assets/181177/486677/ab0f24e4-b930-11e2-9370-72a0c0d85da0.png)

![The item in the context menu of the document's tab label](https://f.cloud.github.com/assets/181177/486678/b98d0626-b930-11e2-8b6d-02f2378016f4.png)

#### TODO/Notes/Future

* Should it have a keybinding? Which one by default?
* Which accelerator label underscore thing to use (if any) for the new menu item labels?
* <del>It might be useful to show a path relative to the project base dir if a project is open in the rename dialog, for example if renaming a file from "src/blah.h" to "include/blah.h".</del> Not applicable now without custom dialog I don't think (unless changing the default filename shown in the Save As dialog).
* It might also be useful to put "Clone" into the Documents sidebar tab and tab label context menus.
* <del>Could get rid of the Save As warning dialog if someone has a decent way to make the Document->Rename menu item from Glade track whether the current document has a file on disk (is saved) and just change it's sensitivity so it couldn't be activated in that case.</del> Not applicable now because the Save As dialog is just shown always and only the default response button is changed depending if the file is on disk or not.
* <del>The word "Rename" is used in a general sense (change the document's name) and not in the other common use as meaning `mv` (rename the file, possibly moving it into a new directory, without copying), so it actually saves a copy and then removes the old file afterwards. The reason for this is that there's already all of the code to update the document state existing and it's easier (and probably even safer) to reuse the `document_save_file*()` functions rather than implement something with `g_rename()`, IMO.</del> All of the existing file Renaming logic is used now, only the menu items are providing some UI queues.
* It seems the Rename button is not used in the Win32 native Save As dialog so this change will do nothing in this case but open the Save As dialog, which is slightly misleading. IIUC the only solution would be to add a rename button to the Win32 native Save As dialog (ugh) or adding several compile-time and runtime checks for whether compiled for Win32 and whether using native dialogs, and disabling/hiding the Rename menu items.
* Would it be better to move the main menu "Rename" item from the "Documents" menu to the "File" menu as mentioned in the one above feature request? Initially it was more "document-centric" with it's own little dialog but now that it literally opens a file saving dialog, it might make more sense in the "File" menu?

If the addition is liked, <del>it would be nice if someone could check the code (mostly in dialogs.c, rest is pretty trivial) and see if I missed some leaks, used utf8 vs locale on filenames correctly for display vs file system names, and also that the actual rename operation is sound from a technical/safety perspective. Much appreciated.</del> I can commit it, the code is pretty trivial now, but a quick review by someone else would still be appreciated.