ENH: Improvements to extension notification icons

Clean up the SVG for the existing extension notification icons, and
change the error icon from a circle to a box (matches oxygen, and
"frees" the circle for the information icon, such that all three have
different shapes). Add an information icon. It's not in use yet, but
will be used for updates.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23320 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Remove unused icons

Remove some icon files from the repository that are not used anywhere.
This is in preparation for a major clean-up and refactoring.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23321 3bd1e089-480b-0410-8dfb-8563597acbee
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)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23322 3bd1e089-480b-0410-8dfb-8563597acbee