Allow any type of plugin to be posted on the marketplace

[krzywon]

As noted in SasView/sasview#2501, plugin slicers would be a good feature to have in SasView. The model marketplace should host more than model plugins, including the proposed plugin slicers, plugin distributions (SasView/sasmodels#271) and other future plugins as they come available. The web interface for each plugin type might need to be slightly different.