2.3.6, Enforce electron31, stop replacing libffmpeg.so

Erequested to merge
(removed):electron31-ffmpeg into main
  1. Stop replacing libffmpeg.so by theia/ffmpeg module. Potentially supports some proprietary codecs. Needed for 2.
  2. Build with electron31 while Theia needs electron30. Fixes #2 (closed)
  3. Minor fix about replacing binaries
  4. Bump pkgver to 2.3.6 since I only succeed to launch with electron31.
  5. No double enter at desktop entry?

Note: No reason that building with electron34 is impossible except for errors about C++20.

Edited by E

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