Bugfixes and improvements

OracleStatementSemanticModel - added  new constructor to initialize
"simple" model which doesn't require a database model. This is useful
when we are not interested in object and function references but just in
a structure of the statements and query blocks
SourcePosition - added ContainsIndex method to avoid complex conditions
Added: OracleNavigationService for navigating over the statement - POC
Added: Navigate to query block root shortcut