Add a possible replacement for GtkEntryCompletion
as a demo.
Move the Dropdowns demo to Lists/Selections, and make
it show both GtkDropDown and the suggestion entry, with
some variations.
This follows the mockups that these demos are
copied from. Unfortunately, it has to be implemented
for every item factory, so we repeat it here.
Fixes: #3291
A dropdown without a model is useless, so accept a model
and expression in the constructor. Allow them to be NULL,
but consume them if given. This makes chained constructors
convenient without breaking language bindings.
Drop gtk_drop_down_set_from_strings() and instead add
gtk_drop_down_new_from_strings().
Update all users.
