Previously this event was not sent for the standard renderers, such as
wxDataViewTextRenderer, at all in wxGTK because the base class
FinishEditing() class didn't do anything if m_editorCtrl was null, as it
was always the case for non-custom renderers.
Fix this by refactoring the base class code in yet another way and
extracting the part which can be reused by both the generic and GTK
implementation in a new DoHandleEditingDone() function and call it from
wxGTK code.
Finally, check "editing-canceled" property to also correctly generate
the event with IsEditCancelled() returning true when editing is canceled
by e.g. pressing Esc in a standard renderer too.
And, as a final bonus, this makes the (just introduced) slightly
artificial DoFinishOrCancelEditing() unnecessary, so it can be removed,
without reintroducing any code duplication.
See #17835.
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