[MonoDevelop] New project model coming soon

[Lluis Sanchez Gual]

Hi!

In the following hours I'm going to commit a big refactoring of the
project model. This new project model will allow MD to use msbuild as
default file format, although the old MD format will still be supported.
Also, it doesn't mean that MD now will use msbuild to build projects.
Projects will still be built using MD's build system.

The commit includes many changes, so there may be some regressions in
existing functionality. I'll do my best to fix all issues asap.

Lluis.