MyGUI plugin for OpenMW resources

Previous attempts to use MyGUI's tools (LayoutEditor, SkinEditor, FontEditor) were not successful because it can not use resources from BSA files. Even after manually extracting them, it still wasn't working, due to backslashes use in path references.

I added a MyGUI plugin that can be loaded by the MyGUI tools and will register Ogre resource groups/archives for BSA archives and the MW data directory, according to the user openmw.cfg, just like it is done in OpenMW. It also registers some custom widget classes used by OpenMW so that they show up as expected.

As always, was only tested on Linux. The install rules need testing on all platforms. I did not add any install rule for Mac, because I could not find any for the other targets either. @corristo?
Why to use `dynamic_cast` if its result is always supposed successful? It is necessary either use `static_cast` or check its result for `nullptr`