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)
COMP: Automatically generate qSlicerBaseQTGUI.qrc

Add build rules to run genqrc.py at build time in order to ensure that
qSlicerBaseQTGUI.qrc is always up to date with respect to the currently
available icons. Embed icon file hashes in the generated .qrc to ensure
that the resource .cxx is regenerated when any of the input resource
files change, while removing explicit dependencies on the files from the
build system (which will break the build when a file is deleted).