Do not restore windows on minimise change

Summary:
On window changing the old code would deactivate the minimise all
plugin, but not restore the rest of the minised windows. This restores
that behaviour.

Also simplify tracking which active window changes, in a way that still
works when a user clicks minimise all then interacts with the desktop.

BUG: 395519

Patch co-written by Anthony Fieroni.

Test Plan:
Clicked minimise all
Clicked desktop
Clicked minimise all, windows restored

Clicked minimise all
Opened a window
Only that window restored
Clicking minimise all again minimised only that window

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13598