[applets/Task Manager] Refine tooltip highlight effect

Summary:
This patch indents the window thumbnail and album art view by one pixel,
ensuring that the edges of the tooltip effect which is drawn under it are
never completely covered up and improving consistency between contents.

BUG: 416390
FIXED-IN: 5.18.0

Test Plan:
Just a window thumbnail:
Before:

After:

Player controls + icon:
Before:

After:

Player controls + window thumbnail:
Before:

After:

Player controls + album art
Before:

After:

Reviewers: Plasma, VDG, filipf, ndavis

Reviewed By: VDG, filipf, ndavis

Subscribers: trmdi, filipf, ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26739