Change default Menu names prefix from 'def-' to 'Default-'

[morgant]

Currently, the default Menu definitions in .mlvwm/MenuBar use 'def-' as a prefix. Since I'm otherwise using capitalized camel case for Menu names, I'd like to change these from 'def-' to 'Default-'.

Of course, this would require also updating a few of the application-specific Menus in .mlvwm/apps/ which are using defaults (currently: BasiliskII, chrome, meld, retroarch, vlc, xcalc, xmag, xterm, scummvm.)

[morgant]

I have updated .mlvwm/MenuBar and appropriate .mlvwm/apps/* to use 'Default-' instead of 'def-', as well as 'Default-Restart' instead of 'sub-restart'.