Filtering SQL expressions for Postgres and shapefile

[Withalion]

Currently all the SQL expressions have been tested just on GeoPackages, but we also want to support Postgres and shapefiles by the specification. Prepare and test the code on Postgres layers. (Shape files should work, but needs to be verified)

TODO:

• single select and multiselect values don't work and crash silently, culprit is IS operator, in sqlite it is interpreted the same as = however in postgres it is used just to check for NULL values (Sqlite docu, Postgres docu)
• pass boolean values as 0/1 instead of TRUE/FALSE (shapefile doesn't support the later variant)