flash: implement 'flash'

Use our own escape sequence for the 'flash' terminfo entry.

Implemented by arming a timer FD and setting a boolean that indicates
we're currently "flashing".

The renderer draws a semi-transparent yellowish layer over the entire
window when "flashing" is active.

terminal.h

@@ -243,6 +243,9 @@ struct terminal {
     char *window_title;
     tll(char *) window_title_stack;
 
+    bool flash_active;
+    int flash_timer_fd;
+
     struct vt vt;
     struct kbd kbd;