Somewhat better encapsulation.

Removes the "false" sharing of Concept implementations, and takes
the opportunity to change the operator==() into a 'canReUseOutputPane'
function to be explicit about its only use.

This doesn't solve the change in output pane reuse behavior yet,
but provides the base to put the required logic into canReUseOutputPane,
as opposed to abusing the general equality concept.

Change-Id: Id9e4e6b8601c5fcf40a252fb423c2c4c2b74ddb6
Reviewed-by: Christian Stenger <christian.stenger@qt.io>