all filebrowser context menu item icons now migrated => LabIcon.
all filebrowser context menu item icons now migrated => LabIcon.

- There seems to be a bug related to how menu items get "recycled"; some icons are getting screwed up after multiple openings and closings of different context menus

jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab