Culverts importer: value map UI

[leendertvanwolfswinkel]

Put a push button in the value map cells instead of the current text edit widget.

If no value map has been set yet, the push button text should be "Set...". If a value map has been set, it can display it in JSON format {"Rond": 2, "Rechthoekig": 0, "Eivormig": 3, "Onbekend": "NULL"} (show only the part of the text that fits on the button).

If user clicks the button, show a dialog to define the value map. The dialog should have:

• Button "Load from source layer". If the user clicks this, the "Source attribute value" column should be filled with unique values from chosen source layer's field.
• A table widget with columns "Source attribute value" and "Target attribute value".
• UI to add and delete rows
• OK / Cancel buttons

[leendertvanwolfswinkel]

@ldebek I have tested this, Can you please make the following changes:

• For the attribute "Friction type", I have specified the source attribute ("type ruwheid") in the main dialog. When I click on the value map button > Load from source layer, I am asked to choose the source attribute. Can you change this so that if the source attribute was already specified, it uses that to fill the value map keys?
• When using "Load from source layer", some keys (source attribute values) may already be present in the value map. It should only add the new keys, instead of the current behaviour that results in duplicate keys.
• I think I have filled in everything correctly, but when clicking RUN I get the message "Please specify a source field for a following attribute(s) and try again:", without the missing source fields specified

[ldebek]

@leendertvanwolfswinkel Requested changes/fixes applied.

[leendertvanwolfswinkel]

@ldebek Sleep-deprived or not, you delivered it :) well done