qmljs: fix race condition in defaultVContext

defaultVContext did call defaultProjectInfo which could
be called only from the ui thread (as it did check the current
project)
Now update the defaultProjectInfo via signals, and lock on access
making defaultProjectInfo threadsafe.

Task-number: QTCREATORBUG-12556
Change-Id: Ibffeb59bbcef7120f08766160bb1e2ddc9af2922
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
QmlDesigner: hotfix for Qt 5.3.2 to ungrab the mouse

At the moment the QML world does not get the release event if
a drag started in QML and ends up on QWidget world.

Hotfix: 3.2
Task-number: QTCREATORBUG-12510
Change-Id: I4f4c023e177221af61ba9e754f27a5087013fb08
Reviewed-by: Marco Bubke <marco.bubke@digia.com>