[applets/taskmanager] Clean up Tooltip code

Summary:
The ToolTip code was kind of messy. This patch adjusts it to use modern Layouts,
fixes some binding loops, reduces redundancy, adds comments, removes a lot of
unnecessary code, and ports almost everything to PlasmaComponents3. We still
need PC2 for `Highlight` so it can't be removed entirely.

There is only one unavoidable visual change, which was made in this patch
because it is consistent with the latest VDG mockup (T12640): the blurred
album art background no longer goes under the player controls bar. This
simplifies the code substantially, and as the mockup shows, it just looks better
anyway (IMO).

This is not intended to fully implement the mockup in T12640.

Test Plan:
Everything looks the same, except for this:

{F8087660}

Reviewers: #plasma, #vdg, ndavis

Reviewed By: #vdg, ndavis

Subscribers: hein, trmdi, broulik, plasma-devel

Tags: #plasma

Maniphest Tasks: T12640

Differential Revision: https://phabricator.kde.org/D27182