[1.0.0-alpha2] Make tree items draggable

Problem/Motivation

TreePanel present a nice hierarchical view of the state tree. However, there is a three-dots-vertical icon which does nothing.

Is it a grab for dragging the tree?

Proposed resolution

Implement a dragging mechanism, using or not the three-dots-vertical icon. Clarify expectations for three-dots-vertical icon.

Remaining tasks

By the way, do we keep both layers & tree panels? Those 2 panels seems to overlap.