More convenience selection of both data attributes and data object

Triggered by @danyill 's initial feedback #2

As a user of the publisher plugin I don't want to navigate in the data model every time I want to select a new data attribute or data object.

Requirement

initialize the finder-list depending on selected FCDA. Load the path of the selected FCDA to the finder
open data object picker if FCDA is a data object
open data attribute picker if FCDA is a data attribute
add new data attribute or data object below the already selected FCDA

JakobVogelsang / oscd-publisher