QmlDebugger: Exchange supported QDataStream versions

Use the lowest QDataStream version common to both the server and client.
The server part was added to Qt 5.0 (commit 7f7cd79e), but we haven't
used that yet.

THis fixes a regression in the inspector where only some properties are
shown.

Task-number: QTCREATORBUG-10186
Change-Id: If6699eb7261e480587f6a21d54fb6b6107669b8f
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>