Use typelib instead of XML

For normal compilation, use .typelib files rather than .gir XML files.
This is much faster.

Rather than using libgirepository, which would try to actually load the
libraries, we use a custom parser.

The language server will still read XML because it needs to access
documentation, which is not in the typelib, but that's generally fine
because it's a long lived process and only has to do that once.

.gitlab-ci.yml

@@ -11,7 +11,7 @@ build:
     - coverage report
     - coverage html
     - coverage xml
-    - meson _build -Ddocs=true
+    - meson _build -Ddocs=true --prefix=/usr
     - ninja -C _build
     - ninja -C _build test
     - ninja -C _build install