BUG: Always use same size for extension icon

Change how we generate extension icons to first resize the raw icon to
the list icon size (and then adjust the rounding radius accordingly).
While not strictly necessary for "normal" sized (128x128) icons, doing
the resize up front saves doing it on each paint, and also gives a more
consistent result for icons of other than the "normal" size. (In
particular, it fixes the text being misaligned in case of smaller icons,
which previously would not be upscaled.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23204 3bd1e089-480b-0410-8dfb-8563597acbee