QSqlTableModel::removeRows() enforce edit strategy

QSqlTableModel::removeRows() enforce edit strategy

For OnFieldChange and OnRowChange, we don't want more than one row in
the cache with uncommitted changes. This could happen if deletion in
the database fails while other changes are pending.

Chosen solution is to return false if other rows have pending changes.
Also, we only allow 1 row removed at a time.

Updated test, changes and documentation.

Change-Id: I68baf6d221789b4754e891535070011c759a2155
Reviewed-by: Honglei Zhang <honglei.zhang@nokia.com>