Each QFileSystemWatch claims its own inotify instance. This is
problematic because there is a default limit of 127 instances per user,
fivolously claiming instances is therefore really poor behavior.

KDirWatch meanwhile shares the same underlying instance behind the scene
so each thread would only have a single instance with N different
watches on it.

Moving to KDW effectively reduces the inotify instance footprint of the
kded module from N per Control to 0 since kded always has a kdirwatch
for ksyscoca/kservice monitoring and our KDW is backed by the same
instance.

CCBUG: 423818


(cherry picked from commit 7223eaa5)