COMP: Don't specify themed icons in .ui file

Remove specification of themed icons from qSlicerAppMainWindow.ui, as
this requires Qt 4.8 (and apparently uic gets really grumpy about
attributes it doesn't recognize, rather than just ignoring them). For
now, we set them in qSlicerAppMainWindow.cxx instead.

See also r23322.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23323 3bd1e089-480b-0410-8dfb-8563597acbee