Rework document controller

While I have tried to keep most of the functionality the same (to get functional changes spelled out in the commit history) there are some significant changes:

When you open a folder or document(s) a default project folder will be based on the path(s) opened. This means that as long as you always open the root of your project (via `mate`, `open -a TextMate`, dragging the folder to TextMate, using the favorites (⇧⌘O), open dialog, `txmt:` URL scheme, or what have you), your project no longer require a `.tm_properties` file setting `projectDirectory` for Find in Folder (⇧⌘T), Go to File (⌘T), and similar to work as desired. Additionally the default properties now set a window title that include the project folder’s name (and SCM branch when available), so most projects can now also drop setting `windowTitle`.

Furthermore the file browser (for when opening documents) will default to the project folder rather than the folder set in Preferences (the latter is still used if there is no document, e.g. after ⌘N or ⌃⌘N).

The default project folder, even for opening a single document, means that opening more documents from the same folder (via Finder or perhaps something that uses the same temp folder for the local versions (which `rmate` does)) means that the following documents will open as tabs in the already open window (containing the first one opened). I _think_ this takes care of the common request of having all documents opened appear in new tabs of the frontmost window.

For managing tabs the ‘Merge All Windows’ action now ignores minimized windows. This means that if you wish to only merge a few windows then you can minimize all but those you wish to merge and then perform the action.

This commit should also address issue #117 (closing window’s last document via the file browser closes the entire window). Canceling a file open also no longer causes the entire window to close, but there’s still some work to do in this department (actually backtracking to previous state, which could mean closing the window if the window was opened only to show this document).

Prior to this commit the tab bar would be automatically pruned when overflowing, this is no longer the case, though this functionality should return.
Render glyphs in Unicode’s “private use area”

Previously we would render these with their code point value but since there are probably a few hundred glyphs available (on OS X) in the private use area, it was a disputed choice.
Use weak pointer for file browser’s delegate