ENH: Clean up icon usage

Create new "XSmall" icon size for 16x16 icons, enabling us to provide
separate 16x16 (e.g. menu) and 22x22 (e.g. tool bar) icon sizes. Replace
ugly undo/redo icons with icons from the (public domain) Tango[1] theme.
Replace home icon with its Tango version for better consistency. Create
a script to generate qSlicerBaseQTGUI.qrc, as manually adding as many as
five entries per new icon is unnecessarily awkward. Arrange to use
themed icons when possible.

(1: http://tango.freedesktop.org/Tango_Icon_Library)