Fix single feature renderer symbol levels reset when changing style

[nirvn]

Description

This PR fixes #26547, whereas the symbol levels were reset to the values set when opening the style panel / layer properties window whenever the layer symbol was modified.

I've taken the opportunity to cleanup the symbol selector {widget, dialog} class a bit.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[nyalldawson]

Can you add a TODO QGIS 4 note here saying "QGIS 4: transfer ownership of symbol", and in the dox note that ownership is NOT transferred and that the symbol MUST exist for the lifetime of the widget

[nirvn]

Done.

[backporting]

The backport to release-3_4 failed:

Commits ["536987c05dac32820a8943b03c24d4edc32560c7","55465f928d1651986be7a2f2bc86c09cd25b3da5","8c7892a2c157642073a7346b885ba29237e394b6"] could not be cherry-picked on top of release-3_4

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport release-3_4
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 536987c05dac32820a8943b03c24d4edc32560c7 55465f928d1651986be7a2f2bc86c09cd25b3da5 8c7892a2c157642073a7346b885ba29237e394b6
# Create a new branch with these backported commits.
git checkout -b backport-31413-to-release-3_4
# Push it to GitHub.
git push --set-upstream origin backport-31413-to-release-3_4
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is release-3_4 and the compare/head branch is backport-31413-to-release-3_4.

[backporting]

The backport to release-3_8 failed:

Commits ["536987c05dac32820a8943b03c24d4edc32560c7","55465f928d1651986be7a2f2bc86c09cd25b3da5","8c7892a2c157642073a7346b885ba29237e394b6"] could not be cherry-picked on top of release-3_8

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport release-3_8
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 536987c05dac32820a8943b03c24d4edc32560c7 55465f928d1651986be7a2f2bc86c09cd25b3da5 8c7892a2c157642073a7346b885ba29237e394b6
# Create a new branch with these backported commits.
git checkout -b backport-31413-to-release-3_8
# Push it to GitHub.
git push --set-upstream origin backport-31413-to-release-3_8
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is release-3_8 and the compare/head branch is backport-31413-to-release-3_8.