[Patch needs rework] Add Map Paint Styles to View menu

[simon04]

In order to achieve #6731, I'd like to add all available Map Paint Styles to the View menu. Available styles can be toggled through the menu, which removes the necessity to view/hide the toggle dialog when this is seldom needed.

What I couldn't manage to get working is to add the new menu entry "Map Paint Styles" to the toolbar for a quicker access. The problem I encountered is that I cannot inherit from JMenu (for submenu entries) and JosmAction (for registration for toolbar) simultaneously. I hope anyone knows how to solve this :-).