Added new user shortcut "scrlck" (SDL/SDL2 using "Scroll Lock" for fullscreen toggle).
Cleanups and improvements in the userbutton shortcut code.
Documentation update (mention SDL2, updated userbutton list)


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12402 6c8e4198-a580-4521-a02f-c9ae86df1db9