xcb: Initialize accessibility lazily, since it requires an event-dispatcher

QSpiAccessibleBridge uses a D-BUS connection, which in turn uses socket
notifiers and timers internally. Neither of these can be used before
a event-dispatcher is in place, so we need to defer creation of the
accessibility interface until later. We assume that clients will
only call QXcbIntegration::accessibility() when an event-dispatcher
is set up, but to be extra safe we do an assert, so that failures of
this pre-condition will trigger at the place they are caused -- not
as failures to register socket notifiers and timers in the D-BUS code.

Change-Id: I4f9d8362a3f285c3da9045d1ff6b8e7b04570488
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Base tst_QGuiApplication on tst_QCoreApplication to increase GUI ED coverage

The QCoreApplication test has quite a few test cases that we would like
to exercise using the GUI event-dispatcher. Instead of duplicating the
tests for the GUI dispatcher, we inherit tst_QCoreApplication, which
also lets us add extra tests that are specific to tst_QGuiApplication.

Change-Id: Ib411457131b8d3fed871f682c1c0568577f6127d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
qdoc: Fixed handling of QML references

Updated qdoc to handle QML references without using the
QML module version.

Task-number: QTBUG-32173
Change-Id: Ibfba9bc92458ae04017706e904625e7d32fc0be4
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>