We need a tree sturcture for rule books

[Bernhard Jung]

It should be possible to define the other books of a regional supplement Produktflöte as sub rule books to the main book.

The sub rule books should be hidden in the optional rules tab where rules are selected.

Selecting the parent rule book should automatically select the sub rule books.

For example the regional armory should be a sub rule book under the main source book for the region.

[Nando Neck]

Issue #202 was marked as a duplicate of this issue.

[Bernhard Jung]

It is now possible to set a parent for a rule book. bffcec8

Only rule books that do not have a parent are shown on the optional rules tab.