Meson: A few improvements
Use gnome.post_install()
It has a few advantages:
- It uses gtk4-update-icon-cache, current script fails if gtk3 is not installed.
- Get path for tools from pc files, they are not always in PATH.
- The script is ran only once even if multiple subprojects use it.
Use gnome.compile_schemas()
This has the advantage to be able to run gnome-text-editor uninstalled using "meson devenv" command.