Make Street Index contents configurable

[hholzgra]

Right now the extra index entries like amenities and villages are hard coded, making these configurable via index style files would be much more flexible and convenient, allowing for new index categories more or less on the fly

• [x] simplify / unify the generation of SQL queries that retrieve index items
• [ ] unify the Python part processing the SQL results
• [ ] map query results to index categories via lookup dictionaries
• [ ] make all this configurable via a simple text format
• [ ] add priority / importance / zoomlevel information for issue #45

[hholzgra]

The three different functions that retrieve and set up the street, amenities and villages indexes have now been refactored to use the same SQL query generating code.

This is a first step towards making these steps configurable instead of being fully defined in Python and SQL code only.

Next steps will be to apply even more refactoring to unify the Python part processing the SQL query results, too, and then the final step will be to come up with a configuration file format for specifying index generators.