QObject: use per-thread storage for qFlagLocation()

qFlagLocation() uses a global char* array to transport source location
information from the connect() side to the metaobject side. The size
of the array is 2 (two), which just about suffices for a single connect()
statement.

Obviously, if more than one thread makes a (_any_) connection at the same
time, the data is useless and, worse, there's a data race.

The non-reentrancy of qFlagLocations() cannot and need not be fixed,
but use a per-thread flagged_locations array in QThreadData so threads
don't disturb each other.

Task-number: QTBUG-3680

Change-Id: If1797c60751f551694def69afee6fbe295bbe2d2
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
QtConcurrent: remove a using declaration in a header file

See Sutter/Alexandrescu, Item 59, for rationale.

Change-Id: Ie689e2a00f9ed0fce1ccfdb77c943dd1b1beb4dc
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Add Q_DECL_OVERRIDE to accessibility classes

This actually shows two completely unused functions in QAccessibleTabBar
(fixed in follow up patch).

Change-Id: If6f464c022617b2ee1db654936e3ff9931103eb6
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>