QmlDesigner.PropertyEditor: Adding QmlModelNodeProxy

We currently use the QmlAnchorBindingProxy in cases that are
not related to anchors. Instead this patch adds a separate object for
ModelNode that we can use instead. This also adds a proper
selectionChanged() signal.

Change-Id: Idbcff3a8efcb3343c9d9b379f1d35ab30b02dc50
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.MetaInfo: Support dot properties defined as components

We can resolve the ObjectValue by using the context and document.

Change-Id: I494737b67e67ce161ef3141356cdb1a380df50ce
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
QmlDesigner.PropertyEditor: Fix ComboBox

We have to wait until the model is properly set until we react
to currentTextChanged. We also have to guard against binding loops.

Task-number: QTCREATORBUG-12257
Change-Id: Id602f6f192962e83a6166bf24a0711b22baa7868
Reviewed-by: Marco Bubke <marco.bubke@digia.com>