Focus events can cause trouble if left on and they can't be turned off

during idle periods (like the other states are) because we'd miss
events. So add a server option to control them. Defaults to off.

tmux.h

@@ -941,6 +941,7 @@ struct window_pane {
 #define PANE_DROP 0x2
 #define PANE_FOCUSED 0x4
 #define PANE_RESIZE 0x8
+#define PANE_FOCUSPUSH 0x10
 
 	char		*cmd;
 	char		*shell;
@@ -1232,6 +1233,7 @@ struct tty {
 #define TTY_UTF8 0x8
 #define TTY_STARTED 0x10
 #define TTY_OPENED 0x20
+#define TTY_FOCUS 0x40
 	int		 flags;
 
 	int		 term_flags;
@@ -1383,6 +1385,9 @@ struct cmd {
 	char			*file;
 	u_int			 line;
 
+#define CMD_CONTROL 0x1
+	int			 flags;
+
 	TAILQ_ENTRY(cmd)	 qentry;
 };
 struct cmd_list {