Summary:
* Unify Desktop and Activity Pager applets. The latter was an out-of-
  sync fork of the former, with 99% code copied needlessly. Separate
  commit to kdeplasma-addons pending that slashes the Activity Pager
  down to a .desktop file.

* Share window state monitoring backend with the Task Manager by porting
  to libtaskmanager. Reduces the window state monitoring in a typical
  plasmashell instance from two to one. Saves CPU time, memory use and
  improves support for Wayland.

* Don't destroy and recreate all delegates every time window focus
  or other window state changes. Saves a lot of CPU time.

* Save various items and objects by e.g. creating window icon and
* desktop
  label items only when needed and cleaning up frontend code.
Lazy-create
  the data source needed to show the activity manager once it's needed.
  Results in much lighter Pager (at least two items per window and one
  item per desktop) with default settings.

* Avoid doing tons of unnecessary calculations during DND.

* Some minor HiDPI scaling fixes.

* Other cleanup: More visualization-agnostic backend, coding style.

Depends on D2674, D2675.

Reviewers: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2676