This is required since Gio 2.80, see [^1].  We however make the
dependency on 2.80 conditional to support older systems as well, as
they don't require the modification to work.

Fixes #119 and mate-desktop/mozo#92.

[^1]: https://gitlab.gnome.org/GNOME/gnome-menus/-/commit/fe1eca74e1b6d75941d56088ea2aeca97b639926

The emit_changed_signal callback was attempting to run on a
submenu that had already been destroyed by its parent menu.

Closes #116

After switching to meson, calling the `g-ir-scanner` command with the
`--warn-all` parameter in the generated `build.ninja` file will display these warnings:

```
Warning: MateMenu: symbol="DesktopEntryType": unknown namespace for identifier "DesktopEntryType"
Warning: MateMenu: symbol='desktop_entry_new': Unknown namespace for symbol 'desktop_entry_new'
Warning: MateMenu: symbol='EntryDirectory': Unknown namespace for identifier 'EntryDirectory'
```

The reason for these warnings is that we use `MateMenu` as `identifier_prefix`
and `matemenu` as `symbol_prefix`, but the symbols that report warnings do not
start with these prefixes.