Allow customizing the text that appears below covers in the cover grid

You can now show any metadata field below the covers in the cover grid
view, not just title. To change the displayed field go to
Preferences->Look & Feel->Cover Grid.
Preload embedded fonts so that the preview panel works off the bat
Fix editing of .txt files broken
...
When renaming, only pre-select the filename part of the name
Remove unused code
Fix keyboard not working on OS X