Initial version of Views-Menu

This is the rivalling change for #18. Looking at it, it might be more a prerequisite. I don't think they will clash, but this one isn't fully finished yet.

What this does, is: Split the `Heaven::ViewDescriptor` into 2 classes. One of them is private and does the descriptor management now. It additionally keeps a checkable `QAction` for every view. This action is checked when the view is visible and unchecked when not. When the user unchecks the action, the view is closed.

Missing is still: When the user checks the action, the view shall appear. But actually, this requires some more work than I initially thought. So I'm probably again deferring a part of a libHeaven task for later.

@antis81 I think, that libHeaven has become an unloved stepchild of mgv lastly. Partially I have problems to remember what work I have begun and what is still completely not started. Add to that that the most concepts of libHeaven are (at least internally) very hard to grasp...