There are many format variables now so allocating all the default ones

each time a tree is created is too expensive. Instead, convert them all
into callbacks and put them in a static table so they only allocate on
demand. The tree remains for the moment for extra (non-default)
variables added by for example copy mode or popups. Also reduce
expensive calls to localtime_r/strftime. GitHub issue 2253.

window.c

@@ -63,17 +63,6 @@ static u_int	next_window_pane_id;
 static u_int	next_window_id;
 static u_int	next_active_point;
 
-/* List of window modes. */
-const struct window_mode *all_window_modes[] = {
-	&window_buffer_mode,
-	&window_client_mode,
-	&window_clock_mode,
-	&window_copy_mode,
-	&window_tree_mode,
-	&window_view_mode,
-	NULL
-};
-
 struct window_pane_input_data {
 	struct cmdq_item	*item;
 	u_int			 wp;