Add major and minor versions to installed files

I didn't do it at first because I thought it would be simpler and more adapted for end-user, but now seems a good time to work that out. It is also very likely necessary to get packaged in distributions someday.

This should address any major and minor combination, but for the sake of required text, I'll stick with 0.3.

[ ] install headers in ${INCLUDEDIR}/valum-0.3 and ${INCLUDEDIR}/vsgi-0.3
[ ] install shared libraries with -0.3 suffix
[ ] install VSGI implementations in ${LIBDIR}/vsgi-0.3/servers (without the -0.3 suffix)
[ ] install VAPI with -0.3 suffix

GIR files are not included in 0.3 and that's ok because we don't compute typelib anyway so it's not really useful. Might be interesting in the future if there could be adoption in Python or JavaScript (with Gjs).

It might be nice to split headers per Vala source for eventual C users, but that mainly depend on Meson.

valum-framework / valum

Add major and minor versions to installed files #184