Editor icons not showing on windows

The icons from the editor toolbar are not visible if the server is running on Windows. This can be fixed by replacing calls to `os.path.join` by `posixpath.join` in `widgets.py`
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Open widgets.py and inspect the os.path.join calls that build the editor icon paths. Replace them with posixpath.join as described, then verify that the editor toolbar icons are visible when the server runs on Windows.

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