Instead of having a customised copy of the default theme in the
app assets, use a custom theme that pulls in the default one and
then overrides things as needed.
In OpenLayers 2.12 the event handling of PanZoom and PanZoomBar clashes
with the result that the zoom level changes by two steps every time one
of the zoom buttons is pressed. Using the Zoom control fixes this issue.
This commit should have no functional affect. It just changes the
abstraction level of OpenLayers from completely abstracting away
getMapExtent etc, to using OpenLayers's functionality directly but
providing proj and unproj to make projections palatable.
