QmlDesigner: Remove custom drag and drop code

Change-Id: I97d94003aef76d014486ff29d0e8f78689f7558f
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Fix drag and drop

Task-number: QTCREATORBUG-12227
Change-Id: I02c3bd81ab2b6585d35bab60550788f461e77732
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Don't reparent on component complete in form editor

It is changing the order of the items which is not intended.

Change-Id: I2af547a1cc5d23e35ec1aa8d1b13c049a5484276
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>