[duim,gtk] Fix usage of rectangles for new names.
[duim,gtk] Fix missing gtk+ property definitions.
[gtk] Missing exports for G_TYPE_* and <GType*>.
[duim,gtk] Fix issues with booleans.

Previously, things that accepted booleans were being passed 0 or 1.
We now use #t / #f.

We also temporarily introduce $true / $false until we have the time
to remove all usages of this and use typical Dylan boolean literals.
[duim,gtk] Fix slot names for events.