Bug 21362: Advanced editor: Fix tab navigation in fixed fields

Bug 21362: Advanced editor: Fix tab navigation in fixed fields

Also minor tweaks to how the fields look.

Test plan:

1. Try moving from field to field and inside control fields with tab/shift-tab.
2. Make sure focus doesn't get stuck in any position.

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit 024643f51c4e0ca1803dd5013bb857e98cd924e5)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit e74f433418e4494c68e996c6d1061d485d67c49e)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>