Experiments with buildung mousepad with meson

[flo.xfce at gmx-topmail.de]

Hey everyone,
Meson is a rather new build system, designed to "not suck". More details
here: https://github.com/mesonbuild/meson/wiki
I took mousepad and added support for building it with meson. You can
find my efforts here: https://github.com/Flow-It/mousepad
If you want to try out how fast it builds compared to autotools you need
meson and ninja installed and this patch for exo:
https://bugzilla.xfce.org/show_bug.cgi?id=12901
After that you need to run (in the source root directory):
mkdir build
meson build -Denable-maintainer-mode=true
cd build
ninja

I don't intend to replace autotools building but just offer a second
option. In general it should not be necessary to touch any existing
files. However, I changed the #include directives because of some
include search path problems and I added a new .desktop template and
renamed an .ui file (all these changes however could be applied to
mousepad without supporting meson, it makes building a bit more
straightforward and increases consistency).

Support for different debug levels (no,minimum,full,...) is currently
not implemented and I haven't tested installing the build yet.

Feedback would be nice. I intend to work on a core component next which
is a bit more complex than mousepad.

Kind regards