Added a NSPopUpButton above the file browser that displays the root path.

Updated the file browser to now display the contents of its root path as the first level, rather than the root node itself.
Selecting an item in the directory pop up button sets it as the root path.
The directory pop up button now has an 'Other…' option that presents an Open dialog box to select a new root directory.
Several fixes and improvements to the directory navigator.

Fixed an issue translating the clicked item to the correct directory.
Improved the aesthetics of the directory popup button.
Double clicking a directory in the file browser now makes it the root directory.
Fixed an issue where directories with repeat names were omitted.
The directory button cell is now theme aware.
Merge branch 'feature-change_directories' into develop

The merging branch adds a popup button above the file browser allowing the user to change its root directory.