Handle "slayer" modifiers on weapons

[dmorcellet]

• implement as "item details"
• import data
• display in items form
• filter (on genus?) in the items explorer
• column in the items explorer

[dmorcellet]

POJO and XML I/O done. Import code done. Display done. TODO : filter, columns, data import and tests.

[dmorcellet]

Data imports and tests OK

[dmorcellet]

Added a column for weapon slayer info.

[dmorcellet]

Done!