Add GObject Introspection annotations

Add GObject Introspection annotations to methods where needed, in
particular adding (transfer none) to return values that don't transfer
ownership.

st_texture_cache_get_actor() and st_texture_cache_get_texture()
are annotated as (transfer none) since they return a newly
created *floating* texture.

https://bugzilla.gnome.org/show_bug.cgi?id=591245

src/st/st-scroll-bar.c

@@ -1095,6 +1095,15 @@ st_scroll_bar_set_adjustment (StScrollBar  *bar,
     }
 }
 
+/**
+ * st_scroll_bar_get_adjustment:
+ * @bar: a #StScrollbar
+ *
+ * Gets the adjustment object that stores the current position
+ * of the scrollbar.
+ *
+ * Return value: (transfer none): the adjustment
+ */
 StAdjustment *
 st_scroll_bar_get_adjustment (StScrollBar *bar)
 {