Renamed some paths and a class for better understanding them:

- Classes in 'databases' were backend classes (doing the hard work) so it got renamed to 'backend'
- Classes in 'wrapper' were frontend classes (which classes from e.g. the "business logic" may use) so it got renamed to 'frontend'
- To both above renames there are already proper "base" classes
- Renamed 'LocalFileDatabase' to 'CachedLocalFileDatabase' as the entire (!) result is stored in $resultData. This is maybe fine for small tables but never good for tables with a lot rows (I don't want to name a number here).
- TODOs.txt updated

Signed-off-by: Roland Häder <roland@mxchange.org>