Small usability improvements to query builder

[heralden]

The Summarise button isn't very obvious, and it also differs from the Summary buttons in the tree. Instead, we could summarise automatically when opening the query builder page, and when changing the root class. This would remove the need for this button.

There's a Clear Query button in the query editor, but it's not obvious that it applies to the model browser as well. If we do the above, we can replace the Summarise button with a Clear button, for an extra button to clear the query.

It may also be worth to consider icons for the three Clear, Expand Selected and Collapse All buttons, with a more descriptive popover that immediately appears on hover.