Add old "Breeze" Task Switcher here under the new name "Sidebar"

Since it's now been taken out of the Breeze Global Theme, which was a
bit of an odd place for it, let's move it here so people who liked it
can continue to use it.

This commit includes the code taken verbatim from kdeplasma-addons as
of commit a4ec95fb8edecab693c26ccd130fa3e99b62322b.

See plasma-desktop#53