Disable comboboxes with no and one item

[MoritzMoreira]

Disable the combobox of all the widget types that can have one, when they contain no or only one item. They appear in the attribute form upon generating new features and in the attribute table. The comboboxes tooltip is set accordingly to inform the user about why there is no option.
As of now, clicking on a combobox with only one item will open it's item list with that one item, changing the whole format in an awkward way and changing the background color to blue. Clicking on an empty combobox does nothing, but hovering over it canges the background color.
The proposed change is mainly for a better graphical user experience, giving immediate information for why there are no other options.

undesired behaviour in case of one item:

Note: The constraint result label (green checkmark) does not behave correctly in many cases, but that is a separate issue)

desired behaviour for hovering over a checkbox with no item:

and one item:

[github-actions]

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[nyalldawson]

@MoritzMoreira i get the rationale here, but I don't like how the widget greys out the field value text when it's disabled.

Are you able to overwrite the widget's palette too so that the text still appears in the normal font color?

[MoritzMoreira]

@nyalldawson I agree, it's better with black test, just added that. Now I will continue to include all widget types with drop boxes, like enumeration in the test, or would it be sufficient to only test value map and value relation?

[github-actions]

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