[browse-perspective] Redesign the tree component

Restyle the tree used in the browse perspective: The tree is everything between (so excluding) the search box and “add root folder”.

The tree should function the same as it does now.
The “bulb” for the menu must also be styled

Scope of this story is documents/images/assets/etc. in the browse perspective. We will create separate stories for occurrences of the tree elsewhere in the CMS.

Preliminary assumptions:

• Language flag is an image overlay, coupled to a css class. Wicket code is used to dynamically set the correct class.

Q&A
Q: what will the selection box look like in case the tree or the document list needs to scroll vertically
A: initially, it is ok that it extends “under” the document list area

Q: the icon in the document list is currently configurable, should we maintain this
A: yes

Q: where do the scrollbars appear (horizontal & vertical!)
A: needs to be addressed in the annotated visuals

Q: which technology do we use for icons?
A: .svg icon files that are referenced in an img tag (<img src="myicon.svg"/>)