Better looking buffering animation (e.g. AdwSpinner)

[heldderarbeit]

When loading a video from a url, clapper currently shows three white dots to show the buffering of new content.

I find these buttons jarring and feel they don't fit the modern style of the rest of the application, especially juxtaposed against the image beneath them.

Since Version 1.6, Libadwaita has now a new spinner widget named AdwSpinner which maybe can used instead:

https://thisweek.gnome.org/posts/2024/07/twig-156/4067bdc35fe1bf7af97eadc464f9b52cfcd8e46b1811437392997908480.mp4
https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/class.Spinner.html

[Rafostar]

NOTE: requires libadwaita 1.6

We cannot use libadwaita widgets in ClapperGtk lib in which current buffering animation is. Video widget must depend on GTK only. The existing animation can always be changed/improved through.