A new combo box

Add a new, simple combo box. The feature set of this is similar
to GtkComboBoxText, but it is using a popover and supports search.

https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/theming/widgets/combobox-replacements.png
Add a test for the new combo box

Add a some simple test app for GtkCombo.
Document GtkCombo

Add documentation for GtkCombo.
Support grouping in the new combo box
Update combo test for new API