This class was vestigial, being a child of PluginPropertiesModel, while
all plugins' properties classes are children of PluginProperties,
which is itself a child of PluginPropertiesModel.

This is a breaking change because external plugins for applications
(e.g. snapcraft) inherit from both PluginProperties and PluginModel.

This does several things:

- Merges PluginProperties and PluginPropertiesModel
- Makes PluginProperties.unmarshal() work for most plugins be default
- Makes PluginProperties.marshal() fully dump json-able objects
- Adds a `plugin` field to all plugins, providing their names.

I missed some changes when I was playing with PluginProperties.
The plugin name will always be a string.

Moves the logic from extract_plugin_properties directly into
PluginProperties.unmarshal()