GtkAssistant API thoughts

[Matthias Clasen <mclasen redhat com>]

Last week I looked at the different proposals for moving GnomeDruid to
GTK+ (see bug 115348). There are 2 patches. The first one has the
following broad characteristics:

- uses ::forward and ::back signals to let apps specify the  
  next/previous page
- has ::prepare signal which let apps override default navigation button
  visibility/sensitivity
- has a concept of page types and uses that to help setting up good
defaults for button visibility/sensitivity
- is just a container which needs to be stuffed in a window
- uses a ::finish signal to signal to the app when to apply changes
- uses ordinary widgets as pages and has child properties for per-page
  data

The second patch 

- uses user-specified functions to determine the next/previous page
- is derived from GtkDialog
- doesn't have a ::finish signal, instead uses the dialog response
  api to signal when to apply changes
- has a special GtkAssistantPage type, and uses ordinary properties on
  it for per-page data
- uses a stack of visited pages for the (default behaviour of the) back
  button

After thinking some more about the different approaches here, I think I
want to try and merge the best aspects of both patches. 

- no page objects in the API, use child properties for title, image, etc
- ditch the "forward" and "back" signals, and use a user-specified function 
  to compute the next page
- derive the assistant from  GtkWindow (I don't think the GtkDialog response
  API is a very good match for an assistant)
- use a stack of visited pages as you do to implement "back". I don't think 
  there is a need to specify a custom back function, it should always go back 
  to the previous page
- keep the page types of the first patch
- add a per-page boolean property "complete", which indicates whether all 
  required fields on the page have been filled in

I think by combining the "forward" function, the page types and the "complete"
information, we can do a very good job with flow control. E.g if all pages
following the current page are already marked complete (because they only
contain optional fields), we can show a "finish" button (if the sequence of
following pages ends in a page of type "confirm", we would show a "last" button
instead, which would jump to the confirm page).