[UI] Sheets and Objects dialog

[stephane demurget free fr]

hi,

First, thanks for Dia, this is an excellent tool. I think it could get even
better (a bit more modern UI, less deprecated stuff). I know contributing
requires a lot of time I most certainly do not have, but still, I'd like to try.

I've developed using Java for the past decade and it's been a long time I've
ever contributed something for Gnome, so going back to C is a bit rough :)

Today, I took quite some time to port the "Sheets and Objects" dialog from
GtkOptionMenu to GtkComboBox.

The question I should have asked first is: what its purpose? It seems to be used
to customize sheets, but I managed to insert a Network object type in the AADL
sheet. This is rather odd, and quite subtile to revert.

Are there more use cases of this dialogs? Wouldn't it be simpler to have some
sort of "Favorites" sheet where one could add an object type in, like editing a
toolbar in Epihany or something like that? It could only be visible if no object
is added to the favorite list.

That would avoid to keep track of the changes of the other sheets, avoid the
startup dialog telling a sheet is customized, and certainly a much higher
complexity of the dialog.

I'm just asking because it might be a better effort than porting the whole
thing.

What do you think?

thanks,

--Stef