[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