Seperate icons from background on panel heading

[jevenski]

1. Oblivion - side panel on mobile

Originally posted by @koblack in https://github.com/Novik/ruTorrent/issues/2721#issuecomment-2403928876

This patch fixes an issue of the heading background of the side panel in Oblivion theme. But under the hood, the change is not limited to the Oblivion theme. The fundamental structure of the headings were altered a bit, for ease of maintenance in the future.

• Previously on the headings of the side panel, open/close icons were defined using the heading background image, which also contains the actual background images for some themes like MaterialDesign and Oblivion. In this patch, the icons are extracted from the background of headings and implemented using the ::before pseudo-elements. This seperates the icons from the heading background images, thus allowing defining heading background using CSS gradient without affecting the icons.
• Dedupe some panel background images in MaterialDesign and Oblivion.
• Fix Oblivion panel heading background width limitation. The new CSS gradient implementation can grow infinitely.

[koblack]

Looks good! A few screenshots from me:

[stickz]

Thanks for the help testing this @koblack. I'm now able to merge this right away!