Mousepad - Update master branch with new code

[Matthew Brush]

Hi,

We should merge `matt/gtksourceview` into `master` of Mousepad 
repository, which would bring in Nick's complete (and awesome) re-write 
of Mousepad (which lives in the `nick_0_3` branch). For anyone not 
following, Nick (and others?) completely re-wrote Mousepad from scratch 
keeping only the general concept (small/fast) and making it into 
something quite current, maintainable and feature-full. Then I took 
Nick's re-write and tweaked it to use a GtkSourceView widget instead of 
a plain GtkTextView widget. Not only did this remove a bunch of code 
which the GtkSourceView widget now takes care of (line numbers, undo, 
and some other stuff) but it also allows pretty sweet syntax 
highlighting for code files (same as Gedit pretty much).

I think at least a handful of people have been testing it with good 
results so it's probably pretty safe to merge it. It would be best if 
someone could review the Autotools changes since I'm not very familiar 
with the Xfce-specific parts of this. I guess it would get even 
more/better testing if the re-write was in the `master` branch.

IIRC, the TODO list includes:
   - Better encoding support
   - Port to use GIO opening/saving
   - anything else?

Any thoughts/opinions/reviews/objections/comments?

Cheers,
Matthew Brush