TreeModel: Add model pointer to TreeItem

... and use the items themselves as main entry point for
item related operations.

With non-uniform tree items it's easier to have item specific
functionality directly in the item implementation instead of
the model.

Change-Id: I4e9b7db98d16b91ddef81917417691129bb83621
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/breakhandler.h

@@ -214,9 +214,6 @@ public:
 
     Breakpoint breakpointById(BreakpointModelId id) const;
 
-signals:
-    void requestExpansion(QModelIndex);
-
 private:
     friend class BreakpointItem;
     friend class Breakpoint;