Change global shortcuts to use the Meta key by default

The current shortcuts could conceivably override app shortcuts, since they don't use the Meta key. The following changes are made:

Switch keyboard layout:               Ctrl+Alt+K   -> Meta+Alt+K
Activate window that wants attention: Ctrl+Alt+A   -> Meta+Ctrl+A
Kill Window:                          Ctrl+Alt+Esc -> Meta+Ctrl+Esc

These are safe to change in the code rather than adding new ones and keeping the old ones around for compatibility's sake because KGlobalAccel::setDefaultShortcuts 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 shortcuts 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