[REPO] Move backends to new Addins/Backends folder

This enables future introduction of Mono.Addins use. Backends would be
considered addins then.

* Each backend is now an assembly of its own.
* Each backend has a MSBuild project file.
* The class Tasque.Application has been made public, since it is
referenced by a backend.
[build,repo] Divided Tasque.exe into libtasque.dll and Tasque.exe

Though already logically diveded into a frontend and a model,
everything has been built into a common assembly. Now those
logical parts have been also separated on the build level and are
built into libtasque.dll and Tasque.exe.

The Tasque.exe source has been moved into a tasque subfolder
to make the separation more appearent in the directory tree.
[repo] Adjust .gitignore to recent changes
[repo] Move RtmNet to new src/Libraries folder

This unifies all source code in the src/ folder.
[repo] Clean up and format nicely
[repo] Moved create-task.py to tests
[repo] Dropped deprecated mdp/mds files
[build] Fix distcheck

* data/images/Makefile.am: Use target uninstall-local to remove
installed icons, not uninstall-hook (uninstall-hook is used for
updating the icon cache)

* po/POTFILES.in:
	- Add src/tasque/GtkTray.cs to translatable files.
	- Adjust file names to repo structure changes.
	- Make sure files are sorted alphabetically.

* Backend-Makefiles: Use explicit source file names. Wildcards
won't do it.
[repo] Rename tasque to Gtk.Tasque

This is a preparation for the introduction of MonoMac.Tasque
[repo] Add AssemblyInfo.cs files to Gtk.Tasque and libtasque
[build] Fix build and debug from within MD
[repo] MSBuild solution: Rename proj tasque to Gtk.Tasque
[i18n] Adjust pot file names to previous path change
[MSBuild] Fix some library references
[tasque] Create app factory project "tasque"

This project provides the executable of tasque and creates - according to
the platform - the specific INativeApplication object:
	- OSXApplication
	- GtkApplication
	- GnomeApplication.

Gtk.Tasque is now a dll, which (in the future) will only serve as the Gtk
frontend library.
[i18n] Adjust po files to previous path change
[MonoMac] Add project stub for MonoMac Tasque UI
[MSBuild] Setup build confs and compiler flags
[MSBuild] Add build project to manage MSBuild build
[build] Fix RtmBackend build