st/adjustment: Ensure changed signal emission is compressed

st/adjustment: Ensure changed signal emission is compressed

The docs say that `st_adjustment_set_values()` emits the `changed`
signal only once but it's actually emitted for each changed property,
this uses the `dispatch_properties_changed` vfunc to emit the `changed`
signal only per call to `st_adjustment_set_values()`. As a positive
side effect this also makes it possible to use `g_object_freeze/thaw_notify`
to compress the `changed` signal emission when using the setters for
properties.

This also fixes the wrong emission of the `changed` signal in
`st_adjustment_set_values()` when only the `value` property is changed.

Side note: the code is heavily inspired by GtkAdjustment

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3023>