virtualdesktops: use Meta key for axis shortcuts
These shortcuts currently don't use the Meta key, which allows them to block app shortcuts (for example, in the Reaper DAW app). In general it's a good idea for global shortcuts to use the Meta key to avoid these kinds of conflicts since apps typically don't use the Meta key for their own shortcuts. Other axis shortcuts set by KWin components already do this, so let's do the same thing here.
BUG: 466915 FIXED-IN: 6.0