Removed project selection dropdown from the filetree (ref #305).
Updated panel captions to be invisible (rev #305).
Reduced sash (border between subsections) in all notebooks (ref #305).
Added floating/docking of notebooks on tab background doubleclick (ref #305).
Disallowed closing Output/Console/Project tabs (fixes #310).
Moved `Project Directory` menu item lower to not activate on OSX (ref #305).
Updated interpreters to check `ProgramFiles` env variable on Windows.

This helps with locating executables for a localized OS.
Updated stack/watch panel captions to be invisible (ref #305).
Switched to AuiToolBar as it provides buttons with dropdowns (ref #305).
Updated application icons (ref #305).
Added `Choose Project Directory` to the toolbar (ref #305).
Added toolbar button dropdown with recent projects (ref #305).
Updated shortcut for Recent File navigation (ref #305).

`Ctrl-<` and `Ctrl->` was a bad choice as it actually requires pressing
`Ctrl-Shift-,` and `Ctrl-Shift-.` on OSX (although it works without
`Shift` on Windows). Updated to `Ctrl-,` and `Ctrl-.` to use the same
shortcuts on all platforms.
Switched to plain background for the toolbar (ref #305).
Removed the gripper on the toolbar (ref #305).
Added applying new UI settings after upgrade (ref #305).
Merge branch 'filetree-simplified'
Added recent files dropdown to the toolbar (ref #305).
Added `Clear items` to the Recent Files menu (ref #305).
Fixed switching to full screen and restoring status bar on OSX (ref #305).