Made About KDE Dialog Public

Some applications(Mostly Kirigami) like Elisa do not use the help menu.
However, they still need an About KDE dialog. For such applications,
this dialog needs to be public.

Might also be useful in plasma mobile.

CCBUG: 442312
3 jobs for !83 with public_about_kde in 1 minute and 21 seconds (queued for 9 seconds)
latest merge request