[Task Manager] Tooltips redesign

Reworks the design of the task manager tooltips and adds new features.

Summary:
* New tooltip design:
** Less screen space occupied by tooltips
** Tooltips have always the same size
** Better structured informations about a task: Appname, window title, desktop,
   activity if useful
** Close button better looking and more visible
* Group tooltips show per group task all individual informations
* Group tooltips provide context menu per task and its group
* Group tooltips in vertical bar are lined up as column
* If supported by media player, controls are available for each instance
  separately
* Fixed: In any case secondary media players instances are always also shown in
  group tooltip
* Fixed: Flickable area when exceeding available display space
* Fixed: Many warning and error messages

Technical overview:
* For groups instead of relying on the window list provided by X, we now use a
  proper submodel of the grouped tasks
* Window titles are processed by reg exp to only show the relevant part of them
* Media player instances are distinguished by their process ids

Reviewers: #plasma, #vdg, hein

Subscribers: plasma-devel

Tags: #plasma, #vdg

Differential Revision: https://phabricator.kde.org/D3738