Add Badge component

KDE software now has several examples of a colored badge with text on top of it, each with a bespoke implementation. It would be better to unify them into a single library component. This merge request creates one.

Kirigami Gallery implementation: sdk/kirigami-gallery!119

Porting things to use it:

• plasma/kinfocenter!262 (merged)
• plasma/plasma-desktop!3089 (merged)
• plasma/discover!1290 (merged)
• plasma/systemsettings!399 (merged)
• plasma/kscreen!456

@broulik @niccolove @sitter @olib @mart @nvrwhere @akselmo @nicolasfella