Merge accelerators in Mozilla .properties files

Fixes bug 115.
Align dtdfile.makeindex() with that of base class

To TranslationStore.makeindex() populates, among others, an “id_index”
dict, while dtdfile’s just populates an “index” and the code in
accesskey module expects the later, so align them together to avoid the
hack in propfile.makeindex()from the previous commit.
Simplify tests

After the previous commits, TestProp2PO.prop2po() can take an optional
personality kwarg, so use that.
Docs: document properties files access key changes