Migrate command shortcuts from khotkeys

khotkeys is finally retired, but users may have a number of existing shortcuts.

While KGlobalAccel has no equivalent for window actions and mouse gestures we
do for command shortcuts, so let's migrate them to kglobalaccel proper.

kglobalaccel already manages these, but the actual execution still resided in
khotkeys.

To fix this, at startup, read the saved shortcuts from khotkeys and create desktop
files for them. This covers command actions as well as DBus actions (since those
can be trivially transformed into a command).

This has the benefit of cleaning out the awkward "Custom Shortcut Service" section
in the KCM.

CCBUG: 480090
(cherry picked from commit 6c6ce651)