Character data: revamp management of trait tree traits

[dmorcellet]

Now, in the character data, the traits selected in the current trait tree are stored directly in the "buffs" section. So:

• trait tree traits are mixed with all other traits and buffs
• plumbing code is necessary before and after the trait tree edition UI

Proposals:

• when loading the buffs section of character data, put the trait tree traits/buffs in a "trait tree status" object.
• those traits are no more loaded into the buffs section. But still used for character stats computation.
• option: store the trait tree status in XML (embedded tag), like it is already done in trait tree files. Caution with this, because old character data files shall continue to be loadable.

This is linked to issue #336 and issue #361.

[dmorcellet]

Almost done. Still testing todo (and warnings to remove) when loading old character data.

[dmorcellet]

Done.