Add API support for file decorators, custom classes and extra columns about tree-finder HOT 1 OPEN

@krassowski tree-finder currently supports all of the things in your list in one form or another. There's probably some conceptual gaps, and there's no nice api for a lot of it, but all of those things are supported.

if file icon is a separate element, then also add/remove class for file icon (to allow to add modifiers with CSS)

I haven't really decided yet how to handle file icons. This will hopefully become clearer as the tree-finder/jupyterlab integration matures. In the meantime, you can use the addStyleListener method of the <tree-finder-grid> element (which inherits the linked code from regular-table and the <regular-table> element) to do pretty much whatever you want to each row of the table, including adding/removing classes.

add/remove non-column decorator

tree-finder (and regular-table below it) don't really distinguish between different kinds of columns, you can even pass in actual live DOM nodes as your column data and tree-finder will just render them. The width of each column is simply the minimum space require to display the contents.

The only thing I can think of that would cause trouble for an "annotation" column is that currently all columns require a name, which gets displayed in the column header. So internally the annotation column would still have a name, and then we could use some styling tweaks to prevent display

from tree-finder.