Change project window size property from two ints to one Vector2i

[timothyqiu]

For 3.x, two ints make sense because we only have Vector2.

For 4.0, I think they can be changed to use one Vector2i. So it'll display the edit control for Vector2i and benefit from features like Vector X-Y value linking.

[Calinou]

One downside is that the inspector does not support min/max range hints for Vector2 yet, so the setter must implement clamping using the CLAMP() macro. It's not a huge downside, just something to keep in mind.

[Mickeon]

I believe, now that exporting properties has been streamlined with annotations, it's time to begin proposing more export options for property types that aren't Integers, Floats or Strings... so that manipulating their setters for very simple operations may not be required nearly as much.