applets/appmenu: fix top-level menu text coloration

28537cf3 made the color dynamic, but
neglected to respect the Plasma color scheme, if any. This causes
problems with Plasma themes that have their own colors and don't
respect the systemwide color scheme, such as Breeze Twilight.

Fix it by using the appropriate colors from the PlasmaCore color scheme
object, not the systemwide object provided by Qt.

BUG: 453348
FIXED-IN: 5.24.6


(cherry picked from commit 19d9bc7e)