mouse: expose three finger drag setting in ui

MR GNOME/mutter!1021 adds
support for three finger drag. This MR exposes the new setting in the
touchpad section of the mouse panel.
11 jobs for three-finger-drag in 6 minutes and 41 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Build
passed build #572145

00:01:26

 
  Test
passed asan #572147

00:05:14

passed test #572146

00:02:51

 
  Manual
manual armv8 #572152
allowed to fail manual
manual flatpak #572148
allowed to fail manual
manual msan #572151
allowed to fail manual
manual ppc64le #572153
allowed to fail manual
manual tsan #572149
allowed to fail manual
manual ubsan #572150
allowed to fail manual
 
  Review
manual review #572154
allowed to fail manual
manual stop_review #572155
allowed to fail manual