Adds a closure to be called by the default main loop when it’s idle.
func will be called repeatedly until it returns
The default main loop almost always is the main loop of the main thread. Thus, the closure is called on the main thread.
idle_add(), this does not require
func to be
Send but can only be called from the thread that owns the main context.
This function panics if called from a different thread than the one that owns the default main context.