Fix AppImage build

[wereii]

changes:

• unlink AppRun, which is symlink to python3.7 binary so that we don't copy our AppRun into it
• rename the appdata.xml to pyfa.appdata.xml - appimagetool kept raising warnings of missing appdata file
• mark dist_assets/linux/AppRun as executable in git (but still chmod it in CI build just to be sure)
• requirements installation: use -f switch to speed up building wxPythong by using official pre-made wheels (though they are not incldued in pypi due to compat reasons) read more here https://wxpython.org/pages/downloads/index.html

Closes #2456

[wereii]

Building of wxPython times out the build but it works on my local™️ after these changes 🎉

[wereii]

@DarkFenX This patch is ready, the main fix is to remove the symlink from AppRun to python binary, https://github.com/pyfa-org/Pyfa/blob/02f61754f79c8984d40c3a74d3dfc3d759af287f/.appveyor.yml#L36

I tried to add some speedups mainly because building the wxPython timeouts the CI for this PR but it didn't work out really and caused errors later on.

[DarkFenX]

Sorry for letting it hang in the air for too long, got caught in a torrent of events irl (and still will be busy for idk how long, at least 1.5 months). I actually checked the patch a while ago and could merge it, but postponed until i went through all the changes and understood what they all meant.

Thanks for the fix.