Fixes intermine#508. There were three main problems:
- subclass was added to any child class of the class (e.g. ThreePrimeUTR
type constraint was added to Gene.UTRs.organism in addition to Gene.UTRs)
- only direct subclasses were selectable, not indirect (descendants)
- it was possible to create a subclass constraint to the original class
(e.g. type constraint for Gene.UTRs to be UTR, which it already is)

- When expanding query tree to a constrainted subclass, dropdown will
  show correct value
- Changing subclass will change it in query editor (when present) and
  fetch preview
- Fix various incorrect docstrings
- Fix bug creating multiple identical subclass constraints causing
  invalid query error
- Remove stale subclasses (resulting in empty class in query editor)
  when removing last view of a class
- Do not allow setting subclass constraint to the class itself (i.e. not
  a subclass)

Since you're able to set subclasses and drill down, without adding it to
:im-query.  Not sure if there's even done a full-path lookup in the
tree, but it's still good practice to make sure the model supports it.

This led to a crash in the query builder caused by setting a subclass
which was empty. The model should be kept complete, and trimming should
be done in the UI.

Now we make sure to add all parent subclasses for an attribute.

- Clear subclass descendants from :enhance-query
- Clear subclass descendant views from :order
- Add parent subclasses when summarizing view

- Couldn't summarize unique class descendant of non-added subclass
  (Flymine e.g. Gene -> Child Features [Binding Site] -> Factor)
- With im-path/walk fixes, subclass constraints changed contents of
  subclass dropdown (selecting dropdown item changed the dropdown)
- With im-path/walk fixes, constrained subclass was displayed in
  parentheses instead of superclass
- Class unique to parent subclass displayed a blank name in Query Editor
  (Flymine e.g. Gene -> Child Features [Binding Site] -> Factor)

Fixes intermine#550. I decided to keep the Summarise button, but add icons,
tooltips and better names.

For this to work properly, tools that wish to support subclasses will
have to make sure their query sets `from` to `imEntity.X.class`.

It used to crash due to lacking subclass support.