eframe should handle sigint/ctrlc signals

[transkatgirl]

eframe's App trait includes the on_exit() function, which is useful for allowing applications which keep track of user state to gracefully shutdown when the application is closed. However, although these respond to the application window being closed, sending a SIGINT signal to the program (or running it in a terminal and pressing ctrl+c) causes it to abruptly close without calling on_exit().

This should probably be handled in eframe, rather than requiring anybody building an application on top of eframe to reimplement signal handling themselves.