EncodingsComboBox: rename COLUMN_ADD -> COLUMN_CONFIGURE_ROW

A better name, since it is to configure the encodings shown in the combo
box.
Improve doc of gedit_encodings_combo_box_get_selected_encoding()
EncodingsDialog: more consistent attribute names

For auto-completion it's nice to type "liststore_" and see the
proposals for the two different liststores.
EncodingsDialog: some code clean-up
EncodingsDialog: use gtk_tree_selection_count_selected_rows()

Welcome in the 2000's!
EncodingsDialog: get_selected_encodings() util function