Added the option to choose between light and dark themes for the file browser sidebar.

TODO: change the path picker color as well.
fixed a small bug when reloading a theme. changed the colors of the nspathcontrol
Now the highlight color in the file browser changes with the theme.
Fixed an issue with the color of the file browser entries after a double click when using the light theme.
New: An indicator appears next to the files browser items that are currently open in vim.

Conflicts:
	src/MacVim/MMFileBrowser/MMFileBrowserController.m