Provide file icons for the resource browser on the client side.

This will allow us to remove QtWidget dependencies on the probe side.
It also enables icons if the target system doesn't have an icon theme
installed at all.
Remove unnecessary dependency on QFileSystemModel.
Add a search line, now that we have a proxy model here already anyway.
Remove QtWidgets dependencies from the server-side resource model.
The probe no longer depends on Qt::Widgets (Qt5 only).