Improve memory management in project window.

* Panel struct does no longer take ownership of its widget.
 * Panels consitently delete their widget in their destructor.

Reviewed-By: dt

src/plugins/projectexplorer/buildsettingspropertiespage.cpp

@@ -73,7 +73,7 @@ BuildSettingsPanel::BuildSettingsPanel(Project *project)
 
 BuildSettingsPanel::~BuildSettingsPanel()
 {
-
+    delete m_widget;
 }
 
 QString BuildSettingsPanel::name() const