Add support for "tabindex" in HTMLFormField subclasses

It is already set for some fields in Special:Block, but are
discarded by HTMLForm and its fields.

Some notes:
- fields with multiple inputs (radio, select and other, select
  or other) will have the same tabindex set on all elements
- Some items such as multi-select and check matrix are not yet
  implemented

Change-Id: I3e1ba7f16f3a3183f231afcf60dd392ce6d6eb6b