Image dialog should not allow zero for width and height fields

[Satya Minnekanti]

Image dialog allows zero to be entered for image width and height. When zero is entered the image is not visible and it is difficult to select and edit the image's properties.

The Image dialog also allows zero to be entered for Horizontal Space, Vertical Space and Border. A value of zero does not cause a visual effect, but an inline style is applied. The inline style should not be applied if it has no effect.

Either validation should not allow a value of zero, or zero can be allowed but it results in no inline style being applied.

Note: Microsoft Word does not allow a value of zero for image width and height.

[Alfonso Martínez de Lizarrondo]

As I said ​previously, a zero value in those attributes besides width and height is perfectly valid, it has the effect of setting those attributes or styles to 0 overriding the default.

[Wiktor Walc]

I have corrected the description so that this ticket could remain valid. The other attributes were discussed earlier in #6488 as Alfonso already mentioned, 0 make sense there.