[C2] show_tabs inconsistencies

[ywarnier]

• The show_tabs setting seems to appear in both categories display and platform (even when it only exists in the "display" category in the database)
• Sometimes it still exists twice in the database (once for display, once for platform) after alpha upgrades
• The title should say "Main menu entries" instead of "Show tabs in the header", because they are not in the header anymore. The description should say "Check the entrie you want to see appear in the main menu"
• Because of that last change, the "show tabs per role" title should be renamed to "Main menu entries per role"

[christianbeeznest]

It is added a migration file to fix show_tabs inconsistencies

c6d5c0c

[ywarnier]

I made a small typo in the text for the description above but it's not worth another migration. I wrote "Check the entrie you want to see appear in the main menu" instead of "Check the entries you want to see appear in the main menu", but I will fix that at the translation level.

[ywarnier]

Working well for me. Thanks!