Add GetWindowSize() function

[ghost]

Adds a GetWindowSize() function that returns the current window size to a Vector2.

This is helpful in case the window size was changed by the user (or the OS, WM, etc). And will be necessary for fixing the SetWindowMonitor().

[ghost]

I'm sorry, I completely missed GetScreenWidth() and GetScreenHeight(). I kept looking for Window and missed that. My apologies.

[raysan5]

@ubkp Actually, it has been proposed several times, many users look for GetWindow*() and ask me why GetScreen*(). Despite it could be less intuitive, I tried to avoid GetWindow{Width/Height}() because some platforms supported by raylib do not use the concept of window, screen framebuffer is "directly" output to display, usually a screen fit on a window (when available) but even on then it could not be the case.

About why GetScreen{Width/Height}() instead of size, two reasons:

1. Usually the pixel size returned is an integer value, Vector2 only support floats and I preferred to avoid adding a Point structure using ints (despite it could be needed on some circunstances).
2. Usage reason, I noticed most of the time it's more useful to have the width and height in separate functions to be queried and used separately. Same happens with some other functions so I tried to be consistent.

[ghost]

@raysan5 Those are very good reasons. Thank you very much for the detailed explanation.