Image preview should have ability to resize itself vertically

[qarmin]

Godot version:
3.1 f2cc969

OS/device including version:
Windows 10

Issue description:
When I click at the image in File system dock, then in inspector is shown this image preview. I can resize this preview horizontally but not vertically, so I can't see more details in image.

[Calinou]

Since the image always has to be displayed in full in the inspector preview, it doesn't make sense to be able to resize the image preview vertically. Otherwise, the dock would have to expand/shrink automatically to account for this, which would feel strange and unexpected for the user.