Added handling of QT_LINUX_ACCESSIBILITY_ALWAYS_ON variable.

By setting this variable to non empty value we can, bypass
checking ScreenReaderEnabled proprty of org.a11y.Status
dbus interface. Not all accessibility readers set this.
The variable is also useful for debugging.

Change-Id: I6c5a1a51065948248e068dfff4755d6818ed9fb1
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>