Start refactoring bonus tests, adding todos for what's left
Add bonus sibling check for PageGroupDescription
All children pages of a bonus PageGroup are bonus
No need for bonus and isLastOfSequence fields in PageGroup

Bonus is set in PageGroupDescription.Position, and in children pages of
pageGroup, but not pageGroup itself. That information is lost at the
PageGroup level when building the sequence, and transferred to the Page
level.
Explicit error a little
Check that not all pages in a pageGroup are bonuses

Because in that case, it must be set at the PageGroup level.
Implement skipRemainingBonuses
Implement isNextBonus and isLastBeforeBonuses in Page
Set isNextBonus and isLastBeforeBonuses flags in SequenceBuilder
Missing utility class from previous commit
Add missing lines creating bugs
Intro in sequence in internal, not public
Fix button text if we're potentially at the last page