Summary:
The look of Breeze menu titles has always slightly bothered me since they have the same
visual style and weighting as items, so they look clickable even though they aren't, and
they don't really do a very good job of separating sections, as seems to be their purpose.
This patch my my attempt to remedy the situation by making them look more "title-like"
and have greater visual distinctiveness from the items above and below them.

Test Plan:
Plasma Task Manager item context menu, before: {F8252758}
Plasma Task Manager context menu, after: {F8253825}

KMoreTools menu, before: {F8252757}
KMoreTools menu, after: {F8253827}

Reviewers: #vdg, #breeze, niccolove, ndavis

Reviewed By: #vdg, #breeze, niccolove, ndavis

Subscribers: cblack, cfeck, ndavis, niccolove, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29081