[Applets/Task Manager] Fix and improve behavior tab layouts

Summary:
Fix the group layout.
Change rest of behavior tab layout.
Rename various labels and texts.

BUG: 415649
FIXED-IN: 5.19.0

Test Plan:
Before:
The indentation of option "Group only when the Task Manager is full"
is misleading.

Before:
{F7882775, layout=inline} {F7882776, layout=inline}

After:
{F7883513, layout=inline} {F7882780, layout=inline}

Reviewers: #vdg, #plasma, hein, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26660