to previous line.

readers like NVDA will speak when extending the selection with a Shift+Down key
combination scrolls but does not move the caret position relative to the screen.

The idle callback removal clause in the ScintillaGTK destructor got
broken in 4e5c321dda8e69104cb7b639c3107dc11dbc00f6, leading to the
callback not being removed thus possibly running after the instance
destruction.
Indeed, gdk_threads_add_idle() wraps g_idle_add() with a custom user
data, thus making the g_source_remove_by_user_data() call in the
destructor incorrect, as we give it our own user data, not GDK's
wrapper one.

Fix the callback removal not to use user data matching to avoid this.