Add a style class for context menus

Add a style class for context menus

Attached widgets inherit from the style of the widget they are
attached to. This can sometimes have unintended consequences,
like a context menu in the main view of gedit inheriting the font
that is configured for documents, or the context menu of the preview
in the font chooser coming up with humongous font size.

To fix this problem, we introduce a context menu style class
and use it for all menus that are used like that. The theme
can then set a font for this style class.

https://bugzilla.gnome.org/show_bug.cgi?id=697127