QSqlTM/QSqlRTM: improve style and readability

QSqlTM/QSqlRTM: improve style and readability

General changes:
const, scope, braces, hash[] for clarity, comment wording and
spelling.

QSqlRelationalTableModel::selectStatement() readability:

Renamed private method.

QVector<Class>.value() already defaults to null object value, so there
is no point in handling this case explicitly.

Alias rec for d->rec added more noise than clarity.

Using "tables" list only adds an extra step. Simple concatenation does
the trick.

Deduplicate code for building table expression and JOIN condition.

Change-Id: Ia52afaf3c3937a26595d5ae867982664002562d8
Reviewed-by: Honglei Zhang <honglei.zhang@nokia.com>