Rework build process

See merge request !2

- to have right paths
- to detect future dependencies that will be needed by some scripts

To generate the AppImage the libraries from the host system will
be used (if it worked on my machine it should works on your machine).

All libraries are copied into the AppImage, it should have a size of
something like 100Mio.

Trying to make the AppImage works on other computers, for now only
works on debian machines, it's facheux for an AppImage

Also add the --enable-appimage that will download the appimage creation tool

For bad or good, having a style enforced by astyle is really usefull

I don't know if it was really a good thing or if it's a starting point,
but I did it anyway.

see changes at https://git.odrling.xyz/odrling/amoegisub

* Look for both system fonts and user fonts on Windows

* Move repeated font registry reading code to separate function

* Pass the files vector to read_fonts_from_key as a reference

* Allocate a larger filename buffer when necessary while reading fonts from registry

Seems like it triggers a download for some browsers...