"popup_centered_minsize" doesn't center the popup

[YeldhamDev]

Godot version:
0181681

Issue description:
When showing a popup using the popup_centered_minsize method, the popup will not be correctly centered. Here is a image with a centered button used to trigger it:

[akien-mga]

I've seen that too (it's pretty visible with the warning dialog I added to the Mono build, which doesn't start properly centered). I read about this issue a week ago on the issue tracker too, but couldn't find the duplicate again, it might have just been in a comment of an unrelated issue.

[vnen]

I have a hunch that this happens because the minimum size is 0,0 by default, and the engine is using that value to centralize, instead of the actual size.

[YeldhamDev]

@vnen Your hunch appears to be correct, did it again, but passed the popup's size to the method, and it centralizes it properly. I always thought that the default values being 0 was for it automatically using the popup's minsize, because otherwise there is no sense for these defaults at all.

Also, I think I found another bug. While it the method squishes the popup horizontally, it doesn't affect anything vertically:

[leopoldek]

I think it might be that the window title size is not being taken into account when calculating the size.
Correct:

Wrong:

[kidrigger]

@Mr-Slurpy Are you working on this? Or can I?

[leopoldek]

@kidrigger You can work on it.