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