Use the GtkComboBoxText API and the GtkComboBoxEntry replacement API
and map those to the old equivalents if not available.

This changes the type exposed by ui_combo_box_add_to_history() from
GtkComboBoxEntry to either GtkComboBox (under GTK2) or GtkComboBoxText
(under GTK3).  This should not be too much of an issue since
GtkComboBoxEntry and GtkComboBoxtext are subclasses of GtkComboBox,
but this will still emit warnings when when the calling code passes
a GtkComboBoxEntry pointer to ui_combo_box_add_to_history().

However, this requires the calling code to use the same mapping as we
do (GtkComboBoxText = GtkComboBox on GTK2, even on 2.24), or things
will blow and it'll be hard to understand why.  This wouldn't be an
issue if the calling code includes our gtkcompat.h header everywhere
it deals with combo boxes, which will be the case if it includes the
Geany headers everywhere but probably won't otherwise.  Oh dear.

A possible kind of workaround may be for ui_combo_box_add_to_history()
to do type-checking on its argument and use the actually correct API
for that type.