File selector enhancements

I set out to enhance the file selector combobox in the IDE, and, well, it led me to make more changes than I anticipated (like redoing parts of file/project closing and saving). Some additions and deletions are due to merging in recent changes to upstream master, which I don't _think_ screwed anything up.

Here's a few new things:

* added a close button next to the file selector that closes the current file. If you hold down shift, it will close all open files/editors
* if you hold Command/Control and hit forward slash, it toggles how file names are displayed in the file selector. Initially, they appear the same as always--the filename.extension. The other setting is to display their path starting with the ProjectName, all the way to the file (e.g. Project/Resources/Sprites/enemy.sprite). I added this partly because sometimes you could have two files with the same name in the file selector, but in different directories or projects. There is one thing I haven't addressed yet, and that's the issue of long strings of text running outside of the combobox.
* you can switch between open files with Command-backquote/tilde or Control-tab. Actually, the former one doesn't work for some reason, so it's commented out for now. So, in the near future, I need to figure that out (or change the key combo on Mac), and I (or someone else) need to implement shortcuts for going backwards through the open files (I have a method in place to do it, just not the key handler). I'll post more about this stuff in [this thread](http://polycode.org/forum/viewtopic.php?f=10&t=530), where I've already mentioned the problem with Command-backquote/tilde.

For other changes, check the commit messages.