[gui] Insure modified attribute form properties are stored when switching from one field to another

[nirvn]

Description

This PR fixes #60181 . I've added a new test class to cover the attributes form properties widget, the regression is pretty bad and we don't want it to happen again silently.

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 7060185)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 7060185)

[qgis-bot]

The backport to release-3_40 failed:

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

stderr

error: could not apply ec32d714d0c... [gui] Insure modified attribute form properties are stored when switching from one field to another (fixes #60181)
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".
hint: Disable this message with "git config set advice.mergeConflict false"

stdout

Auto-merging src/gui/vector/qgsattributesformproperties.cpp
Auto-merging src/gui/vector/qgsattributesformproperties.h
CONFLICT (content): Merge conflict in src/gui/vector/qgsattributesformproperties.h

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-release-3_40 release-3_40
# Navigate to the new working tree
cd .worktrees/backport-release-3_40
# Create a new branch
git switch --create backport-60192-to-release-3_40
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick ec32d714d0ced5dc8a47c4071f8010fb8a8272c2,70601854e10784b26fb24f21ff58385794cbfd0a
# Push it to GitHub
git push --set-upstream origin backport-60192-to-release-3_40
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_40

Then, create a pull request where the base branch is release-3_40 and the compare/head branch is backport-60192-to-release-3_40.

[nicogodet]

A manual backport would be worth. 3.40 is next LTR