GUI functionality fixes.

 - Dimension offsets work well now.
 - Add pen stamp button, to stamp pen attributes on selected items.
 - Fix std::vector references to use a typedef (VPVector for
   std::vector<void *>, etc.) in order to make the code easier to read &
   understand; vector iterators too.
 - Add ability to select all objects.
 - Adding to selection while holding down CTRL with selection rectangle
   works, deselect objects with single clicks as well.