Derive `Default` for MEVENT

[gyscos]

To retrieve a mouse event, we need to use ncurses::getmouse with a *mut EVENT. This means we need to create an "empty" MEVENT to give to this function.
It would be nice to be able to create this object using MEVENT::default(), rather than initializing the fields manually. (The result would be similar to C, where we just declare MEVENT event;)