Enhancement: Tree view icon colors a la the outliner

[autumn-puffin]

Currently the only differentiating factor between node types in the tree view is the icon shape, colouring the icons could help distinguish between nodes at a glance easier, and is already present in the outliner

[Rpinski]

Actually they used to have colors (which can be seen in older wiki screenshots), but with one of the recent releases they all became monochrome in VS Code's default product icon theme.

See also my comment in #684