[kcms/workspaceoptions] Add setting for double click interval
This is part of the legacy mouse KCM, but not the libinput-based one
This is odd because it's not actually a driver-level feature, it's implemented on the toolkit (Qt) level
The KCM only writes a value to kdeglobals that plasma-integration then passes to Qt
It also does not only affect mice, it also applies to touchpads
This patch adds it to the 'General Behavior' KCM, where we already have similar 'Mouse but not acutally mouse' settings like single/double click settings
Edited by Nicolas Fella