Make "Thumbnail Grid" the default Task Switcher and make it live here

Per plasma/plasma-desktop#53, we're
making an overhauled version of the Thumbnail Grid Task Switcher the
default one in Plasma 6.

Currently the default Task Switcher is specified as the "Breeze" Task
Switcher which isn't ideal since it doesn't live in this repo, and it's
possible to use KWin without Plasma, where it does live.

So as a part of making Thumbnail Grid the new default Task Switcher,
let's also move it here so that KWin's default Task Switcher is always
available.

This commit grabs the Thumbnail Grid Task Switcher verbatim from where
it currently lives in the kdeplasma-addons-repo as of commit
54d16f44a56530854444b844536933a3107ef8a6.

BUG: 433034
FIXED-IN: 6.0