Show correct coordinates in statusbar

Retreive the correct coordinates even after resizing the
MandelbrotPanel.
Override equals and hashCode for ComplexGrid
Override equals and hashCode for MandelbrotResult
Add new class MandelbrotZoomer

The features of fractal generation provided by the GUI are shown in
the MandelbrotPanel, but will be controlled by the new class
MandelbrotZoomer.  This is to facilitate testing and to better
separate the GUI from logic.
Update GUI to use MandelbrotZoomer
Add zooming functionality to the GUI
Update resolution of grid when zooming

In case MandelbrotPanel has changed size since last zoom, this will
generate the next fractal with a resolution adjusted according to the
size of the panel.
Fix incorrect index method of ComplexGrid
Fix floating point comparison in Complex

Complex had direct comparison between doubles in the equals method.
This will fail in some cases.  Use a tolerance when checking equality.
Use hashCode of Double in Complex and ComplexGrid

This might prevent collections to treat equal objects as non-equal.