Handle generic item "equipper" effects

[dmorcellet]

Some item categories have "generic" effects associated to them. For instance, shields have "10% Ranged Defence" and some "Critical Defence". We shall:

• load/write these data in a separate file
• avoid writing those stats in the items.xml file
• inject those stats in the Item instances when loaded.

[dmorcellet]

Some of these stats have a 'description override' that shall be translated (see issue #493).

[dmorcellet]

First steps done:

• load data from the DAT files
• read/write data from/to an XML file.

[dmorcellet]

Done!