Add flexible L&F setter class
Add command class to set L&F flexibly
Add menu item for every L&F, fix L&F pref saving

Upon GUI creation (in the Menu factory), create one menu item for each of the
available L&Fs on the system.
When changing the L&F, save the L&F class name into the preference manager
instead of the `LookAndFeelStrategy` descendant class's name; accordingly, when
initializing the GUI, use that class name and pass it to the flexible L&F
setter class.
Move L&F selector menu items into submenu
Remove LookAndFeelStrategy classes

Remove `LookAndFeelStrategy` and its subclasses, since the were no longer
necessary. All of the L&F switching had been handled by
`LookAndFeelStrategyFlexible` already, and it in turn was called by the static
method `OpenDLXSimGui.setLookAndFeel()`. All of the L&F switching functionality
has thus been merged into the method, obsoleting the Strategy classes.
Remove unused name variables