ui: Convert the remaining actions to use symbolic icons

With the toolbar icons converted to symbolic icons we now have a mix of symbolic
and non-symbolic icons in the menu since they share the same GtkAction's.
Convert the remaining actions along with a couple that are placed in the menu
by plugins to use symbolic style icons.

plugins/sort/xed-sort-plugin.c

@@ -84,7 +84,7 @@ static void sort_real (SortDialog *dialog);
 static const GtkActionEntry action_entries[] =
 {
 	{ "Sort",
-	  GTK_STOCK_SORT_ASCENDING,
+	  "view-sort-ascending-symbolic",
 	  N_("S_ort..."),
 	  NULL,
 	  N_("Sort the current document or selection"),

plugins/spell/xed-spell-plugin.c

@@ -88,7 +88,7 @@ static void	auto_spell_cb	(GtkAction *action, XedWindow *window);
 static const GtkActionEntry action_entries[] =
 {
 	{ "CheckSpell",
-	  GTK_STOCK_SPELL_CHECK,
+	  "tools-check-spelling-symbolic",
 	  N_("_Check Spelling..."),
 	  "<shift>F7",
 	  N_("Check the current document for incorrect spelling"),