Added Tool classes

Added Tool classed, that are supposed to differ from actions, in that
they connect to the MPL windows somehow. They are basically made to do
anything that require MPL event notification.
Added mouse wheel zoom

Added mouse wheel zoom to zoompan tool. Will consider adding it to base
class as well, so that it is always there unless it is overridden.
Added Wrapper properties to MDI windows

Added QObject property()'s to the MDI windows, to store owning
SignalWrapper and ModelWrapper. Should enable tracing the entire
hierarchy from any point in it.
Added GaussianTool

Added tool for adding Gaussian components to a Model. Still missing tool
selection indicator (button toggle state).
Made buttons show selected in Toolbar
Improved GaussianTool

Added right-click to remove gaussians, and small cleanups