Add icon to layer and fields comboboxes in "map value" widget's Load …

[DelazJ]

…field from layer dialog

Before / After

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit af0b4fe)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit af0b4fe)

[nyalldawson]

Hmm... any reason why we wouldn't want to use the standard QgsMapLayerComboBox/ QgsFieldCombo widgets here?

[DelazJ]

I simply forgot they exist. Thanks for the reminder. I will do the update.

[github-actions]

The QGIS project highly values your contribution and would love to see this work merged! Unfortunately this PR has not had any activity in the last 14 days and is being automatically marked as "stale". If you think this pull request should be merged, please check

• that all unit tests are passing

• that all comments by reviewers have been addressed

• that there is enough information for reviewers, in particular

  • link to any issues which this pull request fixes

  • add a description of workflows which this pull request fixes

  • add screenshots if applicable

• that you have written unit tests where possible
  In case you should have any uncertainty, please leave a comment and we will be happy to help you proceed with this pull request.
  If there is no further activity on this pull request, it will be closed in a week.

[DelazJ]

I moved the widgets to use the dedicated field and map widgets and much better in the way the preview table is populated. And while at it, I added a help button and dialog geometry restoring.
I've played a lot with that dialog and i find the behavior quite confusing, but didn't address it here. This PR keeps the current behavior. See #59386 for the details on the issue.