GtkComboBox can't be reset

[Diego Zuccato <diego otello alma unibo it>]

Hello.

I Noticed an anti-intuitive thing.
Once I've selected a value in a combo box, I can't "reset" it (from an 
user's POV). Is there a reason not to handle an "esc" press on it to 
call ...set_active(this, -1) ? I know I can do it myself in my programs, 
but IMVHO it should be done by the library.

The patch is trivial, I could send it...

BYtE,
 Diego.