Set starting location for the linkpicker dialog

Essentially this should happen: the starting location of the link picker dialog should be configurable.

The linkpicker dialog opens at the location that is stored in the hippo:docbase property of the facet select. This is not desirable: we do not want to store UUIDs of folders in a facet select besides the root node UUID (cafebabe). It would be better if the linkpicker can be configured with a folder UUID where the dialog opens by default. The behaviour would then be:

1) if a docbase UUID is stored in the facet-select, and it is a document uuid, then the dialog opens in the parent folder of the document
2) otherwise it opens in the configured folder. The uuid of that folder is not stored in the hippo:docbase property.