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
EncodingsDialog: simplify a function

The attribute is named "liststore_displayed", so the new function name
reflects that. It is also simpler to call the function on the 'self'
parameter.
EncodingsComboBox: update correctly the menu

The menu was updated *before* the settings are saved, so the menu was
incorrectly updated. Now the operations are executed in the correct
order: first save the gsetting, then update the combo box menu.

Now the dialog window is destroyed externally (by the combo box),
instead of internally (which is quite strange). So that the
connect_after() works.
Do not use gloabal vars for the command line parsing

Also factor out an open_files helper, this will be used to also
implement application::open in the next commit
Handle also application::open

Even if we do our own command line parsing, we still need to implement
open since the dbus method is used when activating files from nautilus.

https://bugzilla.gnome.org/show_bug.cgi?id=734175
Bump gtksourceview requirement
Rework the headerbar according to the latest mockups

 - restore 3.12 order on the left (since we are not 100% sure let's
   not shuffle things just for the sake of it)
 - use "Save" in text, since usability testing showed that the icon
   is not intuitive
Release 3.13.5
Add candidate-encodings gsetting

https://bugzilla.gnome.org/show_bug.cgi?id=730622