refactoring: remove helper function append_combobox_items()

Its presence was due to me wrongfully thinking that one of the two
classes wxComboBox or wxBitmapComboBox did not offer an "Append()"
function taking a reference of wxArrayString. Tey both do offer those
functions. And using it is a lot faster than appending each item
individually.
mmg's preferenes dialog: reduce time needed for creating it

…by appending combo box options in a wxArrayString instead of
appending each item individually.
mmg's preferences dialog: leave some more space between chapter options
common/wx.h: only declare operator<<(std::ostream, wxString) if wxWidgets doesn't
mmg: let user specify default subtitle character set

See #948.
ChangeLog update
Translations: update Italian
Translations: update list of translatable strings
Translations: update German