Compatibility fix for ExtJS 4.2

When geoext2 is used with ExtJS 4.2, the config option `layout: 'fit'` is absolutely required, otherwise the map won't even show up.

If you run into this issue, it is very hard to spot. So it makes sense to fix this not just in our application, but also on geoext2 side, by using `layout: 'fit'` by default in the main panel class. The attached change does exactly that.