This commit converts `GimpModuleDB` into a `GListModel`. This allows us
to drop quite a bit of custom code to have an adaptive list of modules
by just becoming a `GListModel` implementation.

Next to that, this commit also rewrites `GimpModule` to use the `notify`
signal for its 2 new properties: "auto-load" and "on-disk", rather than
trying to define a custom signal for that. This in turn allows us to use
basic methods like `g_object_bind_property()`.

Finally, the module manager dialog now uses `GtkListBox`, which can
easily bind to that new `GListModel` infrastructure.