Godiva changes:
* Handle custom parameters before init of BaseWMS
* Change LayerSelectorCombo behaviour to not zoom on variable change
* Remove mandatory forcing of version 1.3.0 in MapArea.  This may need
reversing - I can't remember why it was there but there was probably a
good reason.  Whether or not that reason still applies is another
matter...
Made LegendDataGenerator extendable (changed private members/methods to
protected) for use in creating custom legends
Minor bugfixes, new SizedArrowLayer and a perceptually linear rainbow
colour map
Minor Godiva modifications to allow scale ranges in non-converted units
to be retrieved (for screenshots)