doku for comboboxentrytext.h

[Antonio Coralles <noche suapie reflex at>]

the dokumentation for comboboxentrytext.h seems to be newer than the 
latest stable version for the library; at first i was wondering why gcc 
cant find ComboBoxEntryText::get_entry and 
ComboBoxEntryText::set_active_text - but after looking into the header 
myself that becoms clear - these methods aren't declared [i'm using 
gtkmm-2.4.8].

i didn't submit a bug report because i think this problem will 
automatically be fixed with the next version of gtkmm-2.4.