Embed selected version of SDL2 library into AppImage package

[pktiuk]

It would give us better control over libraries used by app. We don't have to rely on packages used by the oldest Ubuntu version.

In current state AppImage version would miss key features introduced in: #416 and #464

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[pktiuk]

@gombosg
Is there any way to update one library in our flatpak runtime?
To maintain proper support for newer features we will have to package the next antimicrox release with fresh SDL2

[gombosg]

Yes, but the we have to build SDL as a module and bundle with the main app. A bit more maintenance burden. Are there any features that only work with the latest SDL version?
https://docs.flatpak.org/en/latest/manifests.html#modules

[pktiuk]

Features:

• storing calibration for multiple controllers in a profile: Allow storing calibration for multiple controllers in a profile #460
• motion sensor support: Implement motion sensor support #416
  Require SDL 2.0.14

But it appears that currently used runtime 5.15-21.08 contains 2.0.16, so we are safe for now.
Sorry for bothering you.

[gombosg]

No problem, let me know if there are some packaging questions!