separate AbstractBasePluginModel and it's base functionality

[benzkji]

ie AbstractBasePluginModelMixin and AbstractBasePluginModel, that form todays AbstractBasePlugin. This would allow to use the basic concept, but without the default fields provided.

The mixin would define a basic API:

• is_visible
• str, with usage of self.to_string()
• add_hidden_flag(text)
• basic css_classes()
• basic html_id()
• html_wrapper_attributes(), using the two above

the model provides:

• basic (oppiniated) fields, like title, published, layout, color, in_menu, anchor
• more advanced css_classes, based on the fields
• some for is_visible
• inherit from models.Model

[benzkji]

note to self: why would this be useful?