Minor code completion improvements

Added: OracleCodeCompletionProvider - schemas and common table
expressions (limited) appear in the object list
Added: OracleCodeCompletionProvider - table join suggestion based on
same column names (used for non-physical objects)
Bugfix: OracleStatementSemanticModel - implicit columns, generated when
asterisk clause is used, are no longer duplicated when a query contains
more than one common table expression
Added: OracleStatementSemanticModelTest - new test set for
OracleStatementSemanticModel class