DatabaseConnection class extracted to encapsulate all db low level code from business logic
redundant create file removed
SQLite concrete types hidden using generic types and interfaces to improve incapsulation
IsEmpty property introduced to indicate that db contains no data
Creating tables encapsulated into separate mothod. DB init fixed
index and table existance checking on create
UI for keep last N files
Setting Keep last N files options implemented
Merge branch 'settings_sql' into last_n_files
fix #26 Keep only N last opened files