Allow getting the group module's drawer state (expanded or not)

[omulh]

I'd like to introduce the idea of making the state of the group/drawer module available for use in the config. or style file.

This could be used, for instance, to change the format of the first element the a group/drawer. Thus allowing a "<" symbol to change to ">" when the drawer is expanded.
See example with current features in the pictures below:

Group/drawer not expanded, the first module of the group is a simple text label with a "<" icon:

Group/drawer expanded, the format of the first module stays the same, a "<" icon:

It would be nice if the "<" could change to ">" in this scenario, to make clear the first module is revealing/hiding the rest of the modules.

I'm not so familiar with Gtk, but it seems the element GtkRevealer has the property "child-revealed", which could be used make this state available somehow.

Thanks!