Filtering land use database based on conditions

[Joonalai]

This PR adds query feature to filter land use database based on certain conditions. Resolves #8.

Features:

• Query any vector layer in the project (should be from same database)
• Show generated SQL query
• Filter layer using substring
• Generate unique choices for fields

UI of the query features

WIP-list:

• [X] There are not that many fields yet
• [X] Show query could perhaps be more sophisticated
• [X] Tests are not using real data yet
• [X] Make abstractions for the most important layers

[Joonalai]

@mvaaltola could you check if the logic is proper here or should we get rid of the "models" and use plain field names etc.?

[Joonalai]

@mvaaltola ready to be reviewed and merged!