ENH: Initial work refactoring extension manager

After a losing battle trying to make the tree widget not use half the
space for the buttons, replace it with a list widget. Change the buttons
to have a stacked layout without enforced horizontal size, which gives
much more space to the extension text, add some padding so the icons
aren't so tight against each other, and increase the font size of the
extension name. Also, replace the random clipart icons with our own.

NOTE: Minimum size for the list items is not working correctly at the
moment. The text rendering needs some additional refactoring in order to
provide a correct minimum size, and a size hint needs to be set
explicitly on the list items.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23166 3bd1e089-480b-0410-8dfb-8563597acbee