Add database storage option

User can enable database storage. If set, two cases :

• tables exist already so he has to
  • define layers names if different from suggested
  • select database schema
  • select CRS
  • attach layers
• only database is existing so he has to
  • define layers names if different from suggested
  • create or select database schema
  • select CRS
  • create tables and attach layers

NB : search_path must be set directly in PostgreSQL