Godot: Allow us to change the color of the viewport border

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Godot version:
All

OS/device including version:
Windows 10

Issue description:
Currently there's no way to change the purple border that represents your window size in a scene. Depending on the color you use for the default clear color, it makes it difficult finding that border when creating your scenes.

You see the purple line, I want to be able to change that. Not sure what it's called.

[Zireael07]

The window border is blue, it's only purple in the upper part because it overlaps with the x axis.
Other than that nitpick, I am in favor of making the color customizable.

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Yes! thanks @Chaosus