l10n: Update Japanese translation
plugin/spell : change deprecated accels methods

gtk_application_add_accelerator() and gtk_application_remove_accelerator()
are deprecated and replaced by gtk_application_set_accels_for_action.
Updated Greek translation
open-document-selector : react to font changes

The list height must react to font changes.
fullscreen mode : center the view on screen

The purpose is to have a distraction free mode.

The GeditViewFrame take account of the right margin visibility to activate
the centering and the right margin position to calculate it, then call
GeditViewHolder to update the spacer

GeditViewHolder is based on Christian Hergert's prototype.
Updated Basque language
Set textdomain as soon as possible

startup is too late and we might get the menus not translated
pythonconsole: replace deprecated method
Cleanup some unused includes
snippets: use gtk_source_file_get_encoding()
Adapt for gtk_source_file_get_encoding() NULL return value

NULL can now be returned.
API doc update
EncodingsComboBox: rename column enums

With auto-completion, it's easier to start with COLUMN so we can see all
the available columns. And for an enum, it's generally better to have a
common prefix.
Add candidate-encodings gsetting

https://bugzilla.gnome.org/show_bug.cgi?id=730622
EncodingsComboBox: code clean-up, improve doc