[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

BUG: 413502