entrycompletion: Various fixes

entrycompletion: Various fixes

Fix various issues that prevented inline completion
and inline selection from working reliably. We were
passing byte counts to gtk_editable_select_region in
one place, but that function expects char counts.
We were listening for GtkEditable::insert-text on
the GtkText widget, but that does not emit those signals,
so listen for GtkEntryBuffer::inserted-text instead.
Finally, we were not clearing the stored completion_prefix
enough, leading to situations where the stored prefix
does not match the text in the entry anymore.