Added limit_sources to RapidDbic and configured CoreSchemaAdmin to use

By default, the CoreSchemaAdmin plugin now only loads grids for
CoreSchema sources which are in use by the current set of plugins of the
app, unless its newly available config option 'all_sources' is set.

The CoreSchemaAdmin plugin is also now confgurable, the 'RapidDbic'
config param can be changed and it will be used for the CoreSchema
model, which will in turn be used by the RapidDbic plugin