Use more compact OSD
A frequent complaint over the years is the size of the OSD. It was tried to alleviate that by having it start fading out slowly immediately but the way it was done wasn't ideal, didn't work on Wayland, and also causes flickering issues in recent Qt versions.
This changes the OSD to a bar-like design similar to the one used in Plasma 4.
Depends on D29263
Various OSD messages
It can grow, if necccessary, to accomodate translations, up to half the screen width.
With Air theme
Full desktop screenshot for some context
Reviewed By: VDG, niccolove, ndavis
Subscribers: ndavis, kori, Armstrong, alexde, achauvel, abetts, ngraham, davidedmundson, hein, Codezela, filipf, zzag, plasma-devel
Differential Revision: https://phabricator.kde.org/D20569