ProjectExplorer: Hotfix for crash on session switch.

In ProjectTree::update() as well as in
CurrentProjectFilter::currentProjectChanged(), there are crashes due to
accesses to a pointer to a deleted project. Wrap the pointer in a
QPointer to prevent this.
May or may not be the correct long-term solution, but does fix the
crash.

Change-Id: I4182a4753578cc4765b9120a36cd8701d07c94c1
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>