do not use field alias for the display role in the field mapping model (fix #53028)

[alexbruy]

Description

Field mapping model uses a field alias for display role, as a result even if user changes field name they won't see a new field name in the widget. This is confusing and might look like it is not possible to change a field name.

It is proposed to use field name and alias for display role, this way changes in the field name will be immediately reflected in the widget and at the same time fiels alias will provide more context.

Fixes #53028.

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 95cf3e4953... do not use field alias for the display role in the field mapping model (fix #53028)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

stdout

Auto-merging src/gui/qgsfieldmappingmodel.cpp
Auto-merging tests/src/python/test_qgsfieldmappingwidget.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgsfieldmappingwidget.py

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-53365-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 95cf3e4953eacca26aa1ab057211635839b03b3a
# Push it to GitHub
git push --set-upstream origin backport-53365-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-53365-to-queued_ltr_backports.