* Project updates

- The WPF projects now correctly respect the local directory.
- The WPF projects now use the premake5 WPF-flag.
- Added missing excludes to .gitignore.
* Typo (fixes missing files in WPF Editor Controls project)
* Major rework towards MEF and AvalonDock

- Extensions are now handled through the Managed Extension Framework (MEF) instead of a custom solution.
- Added support for AvalonDock to support docking, etc.
- Much of the editor code has been updated to support both these cases or simply to clean up.

Note: Command line version of the tools isn't working at the moment.
Note: Some more cleanup is likely needed, but there were already many more changes than I was happy with in a single commit.
* Removed some more legacy code from extensions
* Added basic support for menus.
* Implemented support for opening a project file, listing the project content and editing the project file.
* Added missing license.
* Added module support.

It's now possible to add modules that will be added to the solution. This allows for less clutter in the primary premake file and a higher degree of configuration of mostly the editor. As an example an image viewer has been added.
Project implementation is now an extension.
Moved filesystem project related files into their new module.
Moved menus from the generic project interface to the specialized project implementation.
Moved the new project module into the proper namespace.
Renamed IEditorDescriptor to IProjectItemDescriptor so it can cover more functionality.
Applied IProjectItemDescriptor where needed.
Extended IProjectItemDescriptor with support for querying if new instances can be created and to create them.
Added a version of RelayCommand without arguments.
Added support for closing windows/dialogs in the window manager.
Added support for adding new project items to a project.