applets/taskmanager: Make group indicator icon follow accent color

Tem PQDrequested to merge
tempqd/libplasma:accent-colored-group-indicator into master

The green plus icon is pretty eye-catching, and normally green plus buttons indicate something is going to be added or created. Change this to follow the user-selected color scheme, also matching the focus/active indicator color.

Before After
Breeze Light before-breezelight after-breezelight
Breeze Dark before-breezedark after-breezedark

And some other colors for fun:

Red Yellow
after-red after-yellow

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