Allow a theme to override the form factor of the tabs to be mobile

[andydotxyz]

Continuing from the unfortunately-merged-unmerged https://github.com/fyne-io/fyne/pull/5225

Checklist:

• [x] Tests included.
• [x] Lint and formatter run with no errors.
• [x] Tests all pass.
• [x] Public APIs match existing style and have Since: line.

[andydotxyz]

Requests were fixed 2 weeks ago, merging