Fix coverart tootip if cover is not cached

[daschuer]

This is on to of #12009 on the way to finally fix #11131

[daschuer]

This should fix the issue on Windows as well, reported here: #12103 (comment)

[JoergAtGithub]

In general this works as intended, but I noticed:

• the delay until the tooltip appears initially (this has always been like this - no regression)
• this flickering if I hover very slowly over the coverart column in the table (it looks like the tooltip is redrawn at a new position, before the image is fully drawn):

FlickeringCoverArtTooltip.mp4

This is no a change request, but you might have an idea, where this comes from.

[daschuer]

I think you see the stacked cover fetch events they all result in redrawing the tool tip. Can you please test with #12103 which is on top of this. There is a chance that the flickering is fixed with it.

[JoergAtGithub]

The flickering appears also with #12103

[JoergAtGithub]

This fixes the issue, that the tooltip is not shown after loading the cover art into the cache. But I noticed other cases when the tooltip is not shown. But these affect also other (text) tooltips, therefore these cases are unrelated.
Code LGTM! Thank you!

[daschuer]

Thank you for investigations.