Port to meson, gda5, eds-3.33

This MR combines (old version of) !5 (closed), !6 (closed), !7 (closed) and obsoletes !1 (closed) and !3 (closed)

Replace autotools fully with meson, dropping autotools in the process. Takes !7 (closed) and works on top of it to bring more or less feature-parity with autotools build, with comparable end result between a autotools install before this MR and a meson install after this MR. Also takes the fixes referenced in !5 (closed) and makes it compile with evolution-data-server-3.33+ (libecal API break) as well. I did not drop the code yet, as maybe I'll look into fixing it at some point; but for now, it at least builds and the meson option and summary heavily stresses that it's broken/experimental. The eds backend (as opposed to the simpler thing that almost built) is not hooked in at all, as it is completely broken anyhow. The source dir is kept for now, should it be revived or decided to not be revived and can be handled separate of this MR. Ancient .NET bindings get deleted by this MR.

The install-guide is not updated to the new build system - seems more like a deletion or rewrite candidate to me.

The user-guide is using old OMF stuff, so I figured it'd be better to rewrite it into Mallard instead and then hook that up instead. I'd imagine the old version content would be rather updated by the time any release would be coming out anyways.

Additionally I noticed some imported eel source files do a HAVE_RENDER check, but nothing checks for it and defines it, but this was the case with autotools already.