Add support for app folders

[apcamargo]

It would be nice if we could create folders to organize the apps in the launcher. Here's a quick mockup that I've done to represent a possible implementation of these folders.

[chances]

This should also support how Google Chrome uses ~/.local/share/desktop-directories/chrome-apps.directory to store Chrome Apps:

[Desktop Entry]
Version=1.0
Type=Directory
Name=Chrome Apps
Icon=google-chrome