Cleanup tool loading.

* GLWidget::setActiveTool/setDefaultTool will add the tool if not
  already in m_tools. This makes it easier for simple use cases
  to set up the GLWidget (e.g. MongoChem only needs Navigator).
* Documentation
* GLWidget explicitly takes ownership of its tools.
* GLWidget now calls ToolPlugin::setGLWidget() itself when a tool
  is added.

Change-Id: Ic898919978acea86fbe71e56967276731ed947c9
Merge topic 'glwidget-tools' into master

54551a7b Cleanup tool loading.