Fixed WHERE conditions handling

Summary:

• Fixed possible issue: the WHERE condition can be lost while converting query to SQL This can be seen in Kexi Query Designer.

• Query: validate and offer error message/description while setting or extending WHERE conditions This helps to find table/column names for WHERE expressions in Kexi Query Designer.

+ improve docs

• Query parsing: try harder to locate positions of table/table aliases This fixes stability of Kexi Query Designer

See https://bugs.kde.org/377579

Test Plan: Run all KDb tests

Reviewers: piggz

Tags: KDb

Differential Revision: https://phabricator.kde.org/D5294