Improved MonoDevelop location detection

Properly detect the MonoDevelop location when the add-in is being
built as part of MonoDevelop.
Directly execute mdtool.exe instead of using the script

Required since when building the add-in as part of MonoDevelop
there is no script to run
Use Xwt from the installed MonoDevelop when available
Added special case for detecting MonoDevelop location

Required when building the add-in from inside the MonoDevelop
build directory.
Fix Xwt.Gtk location in the MonoDevelop add-in project
Fix build issue in the MonoDevelop add-in

Add Makefile as a dependency of the generated project files.
This ensures that the project files are always regenerated
when the configure script is run
Don't copy the .addins.xml file to the output directory

The .addins.xml file is already embedded in the add-in assembly,
so there is no need to copy it. The add-in package can be built
using the add-in assembly as source file.
Don't local copy the gtk# assemblies
Merge pull request #206 from slluis/build-fixes

Added support for building the MonoDevelop add-in using a local source directory of MonoDevelop