Show the unit profile based on the variation_id.

The old implementation took the variation_name which is now used to show
the user a translated text.
variation_id replaces the former variation_name.

The new [variation] attribute variation_id is now used to identify the
subtype of the variation. This was done by the variation_name until now.
variation_name is now used to display a translated string to the user.

During creation of the unit tree a new attribute base_ids is stored to
hold a list of the [base_unit]s.
Don't show active "disable" special weapons in the gui1 attack dialog.
Show siblings and base units in the unit profile dialog.
Insert some scopes to make the code more easy to read.
Basic support for base_unit sections.

Units which are base_units for variants are now displayed as sections
with their variants as topics in the help dialog.
Support for the disable weapon special.
Updates to the unit help browser.

Don't display base_units and variations twice.
Fix some linking according to the new situation (sectioned parents).
Changelog updates, related to help browser and unit variants.