Change default keyboard switching shortcut to Meta+Alt+K

The current shortcut is Ctrl+Alt+K which would conceivably override app shortcuts, since it doesn't use the Meta key.

This is safe to change in the code rather than adding a new one and keeping the old one around for compatibility's sake because KGlobalAccel::setShortcut by default preserves old shortcuts when they are changed, and we aren't overriding that behavior here. So only new installs will get the new shortcut and existing user installs will be untouched.

I tested this and verified that the old shortcuts still work, and the new ones don't. If folks would prefer that the new shortcuts also work for existing user installs, I can add them in addition to the old ones.

@teams/usability