Add support for focus notifications when tmux pane changes, based on work by

Aaron Jensen.

tmux.h

@@ -665,6 +665,7 @@ struct mode_key_table {
 #define MODE_MOUSE_UTF8 0x100
 #define MODE_MOUSE_SGR 0x200
 #define MODE_BRACKETPASTE 0x400
+#define MODE_FOCUSON 0x800
 
 #define ALL_MOUSE_MODES (MODE_MOUSE_STANDARD|MODE_MOUSE_BUTTON|MODE_MOUSE_ANY)
 
@@ -926,6 +927,7 @@ struct window_pane {
 	int		 flags;
 #define PANE_REDRAW 0x1
 #define PANE_DROP 0x2
+#define PANE_FOCUSED 0x4
 
 	char		*cmd;
 	char		*shell;