New GdlDock architecture

[Gustavo Giráldez <gustavo giraldez gmx net>]

Hi all,

After almost a month of hacking I finally finished (well, not quite) the
new GdlDock.  It has a completely new architecture, even though is like
95% source compatible with the old API.  The changes to anjuta2 are also
in place so you don't need to worry about that.  One thing that has
changed is the format of the XML layout file, so you'll probably get an
empty screen when launching anjuta2.  Either you remove
~/.anjuta/layout.xml or just open the layout manager and check all the
visible checkboxes.

There is a file called README.gdl-dock in the gdl module which you can
read to learn a bit about the new architecture.  It allows things like
docking items to floating items, but the changes are mostly not
visible.  The design is a lot cleaner, which will allow us to extend the
widget more easily.  For example, layout saving/building is now
completely separate from the dock itself, and it's handled in a separate
object.

There is one thing that still needs work: placeholders.  Even though
they work for simple things such as hiding and restoring an item's
position, I expect them to be a reliable way to dock a new item in a
given position, independently of how the current items are docked.  The
code is simple right now, so it doesn't handle special cases very well. 
Anyway, it's failsafe, so the worst thing that can happen is that new
items float instead of being docked to the main window.

That's it for now.  Please, test and file bug reports to bugzilla.

Have fun,
Gustavo