Scalebar label cleanups

[nyalldawson]

A couple of small tweaks as a lead in to some later scalebar improvements:

• Use a single combo box for scale bar horizontal/vertical label placement. Since we only have 2 choices for each, we can reduce the number of widgets by just exposing 4 choices in a single combo.
• Promote scalebar enums to enum class, move to Qgis

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 82d2189)

[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9225
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!