Remove leftover #merge method from Tag::Enum.

Was forgotten in at previous refactor.
Eliminate :name from :member_type definition.

Instead use the already defined :tagname field.
Avoid NilClass error when rendering parameters.
Remove old unused images from Docs app resources.

A bunch of these were used in the old Touch welcome page, which
no more exists.

Some were leftovers from various style changes throughout the
Docs app history.
Separate member icons from various other icons.
Avoid already defined constant warnings in Ruby 1.8.

In Ruby 1.8 it happens that MemberTag is loaded twice and the
constants in it get redefined, and trigger a warning.
Move member icons to separate image files.
Remove old unused TagRegistry#member_types param.
User-defined member icons.

Member icons are now defined in member tag classes.  There the
:icon field defines the icon file which is included to the docs
app resources and an appropriate CSS gets generated.