PropertyLinkItem now support all major types of link property.
PropertyLinkListItem is no longer necessary, but kept for backward
compatible, which is now identical to PropertyLinkItem.
DlgPropertyLink, the link selection dialog, is now made modeless, so
that that user can select geometry sub-element directory from 3D view.
* Display property from linked object, colored green,
* Change DlgPropertyLink to support external linking and sub-object
selection
* Improve large selection performance by using a timer
* Improve TAB key behavior in property editor
* Add context menu to show hidden properties, change property status,
set expression on any and property, and add/remove dynamic properties
* Optimize expression completer model construction, as the original
implementation gets prohibitively slow for moderate number of objects.
