Various OSD improvements

Make only OSD background translucent, keep text and images opaque

BUG: 187781

Render OSD images with higher DPI for sharper scaled display performance. Also clean up some code.

Change OSD rating painting to use the general KRatingPainter. Moving onwards from the funky yellow OSD stars is a big step, but seeing that it was my patch that introduced them into OSD 18 years ago (commit 7b503586), I guess it is OK that I also remove the code.

Show track progress in OSD. Kind of solves

BUG: 278445

which asks for a way to see position in playing track without bringing up Amarok, which this does - although not via system tray tooltip as suggested on report.

Before:

After:

And –'ified: