Added "new File" functionality

Currently, no new file is associated with the empty editor and a
temporary file will be created when the user "assembles" the code.
If the editor content is saved (before or after assemble) a file will be
created.
Refactoring (issue #9)

Pulled hashCode() of objects for EventCommandLookUp into its get() and
put() methods.
Early fix for issue #11

Added hash code to represent the saved editor state to prevent useless
save dialog on "New" file. Further, setSavedState() currently might not
be used consequently.
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.