More porting of the dock library to GTK 3. The dock panels are (mostly)
appearing now!

- Migrate another GetPreferredSize to GetPreferredWidth/Height.
- The previous default parameters to PackEnd were (true, true, 0), so
  I've switched to that.
- Fix a few issues with the DockContainer.OnRealized function, based on
  what Banshee did in their GTK 3 port. This fixes a ton of warnings
  that were being displayed while starting up Pinta, and the dock
  widgets are now starting to show up.