Completely rework currentNode/currentProject handling

Centralize the handling inside a new class ProjectTree.
React to moving focus and remove most special handling.

This properly fixes the linked task.

Task-number: QTCREATORBUG-13357
Change-Id: I6b06aa32b1e4305ec8a6d432857b302585d8734b
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>