GLFW Window Focus Fix - #8530

Ideally this should be a windowResized event in coreEvents, &ofBaseApp::windowFocusChanged... however I think for complexity it's easier for the moment to just add it as a ofEvent windowFocusChanged where opt in is:

ofAddListener(ofEvents().windowFocusChanged, this, &MyApp::onFocusChanged)

easy

Added ofEvent windowFocusChanged; to ofEvents

@ofTheo is ofEvent good idea for now vs change core events virtual function etc... I think this is good to merge

Thanks @danoli3 I'm not using this anymore in my day to day use.
but I've been using a simplified version of this for the last two years
if you want to take a look it is this code:
https://github.com/dimitre/openFrameworks/blob/ofworks/libs/openFrameworks/app/ofAppGLFWWindow.cpp

I removed some code that counted frames since state change and others simplifications. but there is api change there so near zero change of being pulled to OF main :/

Other two niceties there is ofWindowSettings is only one object for all kinds of windows (I think it is best) and also we can choose to span one same window to multiple monitors (poor's man Nvidia Mosaic) but I think it is very useful to span 4 outputs in one single window, as I do often.

I'm going to put this in for now as Focus / pause event quite useful in general for apps for power usage