Improve show_page_of_new_directory()

What a horrible name! It is neither necessarily a directory(group) (can be an Entry) nor is it a new one. This is also used to display all existing pages..

This MR contains a series of patches that build upon each other but are designed so that the whole thing should still work after each patch (so that e.g. bisecting keeps working).

As a bonus we remove the manual creation of a Hdy.Clamp from python code simply into the glade.ui. This significantly simplifies the important operation of displaying new pages and will us allow more simplifications e.g. merging this with the self.show_element(group|entry) function which does something VERY similar.