Dark and animated MediaViewer

This is part of my GSoC project (!1112 (merged)), but since this task is quite self-contained, I've created a separate MR.

This is definitely not perfect and it also requires changes in some other widgets in order to fix some of the problems.

Things that need to be addressed:

• Some message media (the ones that do not occupy all the available horizontal space) have a stretched animation. This is due to the ContentMessageMedia allocating extra space, so it probably needs a fix in its measure/size_allocate implementation.
• Return early in the snapshot implementation when the widget is not animating.
• Remove the spinner state of the MediaViewer and use the media thumbnail instead (not too sure if that's doable though).

Preview of the current state:

2022-08-26-23-04-32