Add a global color option to the simple config

The goal is to be able to change the color of several objects with a single option (in the simple configuration window).
For instance, menus, dialogs, dock and labels could share the same options for background color, outline color, and corner radius.
Outline color could even be the same as background color, but fully opaque (alpha=1)
The color option should have a "default" value, in which case it will try to pick the color from the GTK theme (from menu's background color, or window's background color).