GLFW Window Focus Fix

[danoli3]

ofxGLFWWindow Focus

• Add bWindowFocused member to ofAppGLFWWindow
• Add focus_cb static callback (matches GLFWfocusfun), registered with
  glfwSetWindowFocusCallback alongside the other window callbacks in setup()
• Seed initial state from glfwGetWindowAttrib(windowP, GLFW_FOCUSED) right
  after registration, since a window isn't guaranteed to start focused
• Clear the callback in close() for symmetry with the other teardown calls
• isWindowActive() now returns bWindowFocused instead of the stub

ofAppGLFWWindow::isWindowActive()was hardcoded toreturn true`, so window now can be detected if active or not active... useful for backgrounding apps running machines, so you can now actively pause or sleep draw or update

[danoli3]

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

[danoli3]

Added ofEvent windowFocusChanged; to ofEvents

[danoli3]

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