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.
22;0|1|2t pushes the current window title/icon to the stack, while 23
pops it.
The second parameter, 0|1|2 has the following meaning:
0 - push/pop icon+title
1 - push/pop icon
2 - push/pop title
