gtk_entry_completion_set_text_column: reuse old renderer

gtk_entry_completion_set_text_column: reuse old renderer

gtk_entry_completion_set_text_column() always added a cell renderer,
regardless of whether there was an existing one already installed.  This
patch reuses an old renderer if it exists, but only if it was added by a
previous call to this function.

To avoid conflicts, all renderers that were added manually are removed
when calling this function. Also, the renderer added by this function is
removed when manually adding new renderers. This effectively gives
GtkEntryCompletion two modes (managed and manual cell renderers) and
allows seamless switching between the two.

This is a minor API break. However, this shouldn't be an issue in
practice as applications couldn't call set_text_column() more than once
because of this bug. Also, it is unlikely that many applications mix
set_text_column() and custom cell renderers. The interaction between the
two modes was erratic and not documented well.

https://bugzilla.gnome.org/show_bug.cgi?id=635499