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