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