Release all pressed keys when the Bochs window gets back the keyboard focus.
- feature implemented in the wxWidgets gui
- feature not required for legacy SDL (releases keys on focus change)
- removed duplicate code in keyboard.cc
- TODO: implement feature in the win32 gui if required


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12582 6c8e4198-a580-4521-a02f-c9ae86df1db9