Add window.get_window_size (DEF-3012)

[britzl]

From Github issue; [(LINK REMOVED){quote}We can get window resize information as windows are resized, but can't get initial size from Window API
{quote}
2019-04-26 16:57:12 (Johan.Beck-Noren)
Today, there is different behaviour depending on platforms. iOS calls listener for both size & focus att app init, and android does not. We need to agree on the behaviour we want across all platforms and then implement it. Design.

[britzl]

Another solution would be to always call the listener when it gets set