Add a gsettings for the last wrapping split -mode

https://bugzilla.gnome.org/show_bug.cgi?id=726739

The last split mode is store in a static var
but if we set wrapping mode off, this state is lost.

So, we add a gsettings named 'wrap-last-split-mode',
under org.gnome.gedit.WrapMode,
who can take GTK_WRAP_WORD or GTK_WRAP_CHAR value
to keep track of it.
Statusbar: line/col popover menu

https://bugzilla.gnome.org/show_bug.cgi?id=726739