New class: KNSWidgets::Action

Introduce KNSWidgets::Action class. Same behavior as KNS3::Button but inherited from QAction instead.

Further merge requests are pending

Deps: plasma/plasma-workspace!1184 (merged)

plasma/kwin!1624 (merged)

education/cantor!34 (merged)

multimedia/k3b!18 (merged)

education/kanagram!6 (merged)

education/khangman!4 (merged)

games/kigo!4 (merged)

education/kstars!469 (merged)

education/kturtle!7 (merged)

education/kwordquiz!7 (merged)

games/libkdegames!7 (merged)

education/parley!18 (merged)

education/step!5 (merged)