Add new component for the greyed out labels in Item Delegates.

Summary:
Plasma code is full of Label {opacity: 0.6}. We shouldn't be hardcoding
style into a UI, especially with magic numbers.

This adds a new component that does the same thing but will allow
plasmoids to write more semantic code.

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: hein, sebas, broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4831