Create Look&Feel menu items dynamically for all installed L&Fs

As discovered in issue #6, the "System" and "Java" L&F which are currently available via the `Window` menu can sometimes be the same. In such a situation, the user will not see any change and will not get any feedback about the "change" of L&F.

These changes modify the L&F menu items in such a way that they are dynamically created: one menu item for every installed L&F.

The current state is a working proof of concept. Changing the L&F works, but inconsistencies might be created (e.g. the selected L&F is not saved/loaded in the global config yet).