Support the default_page_template_title filter in page-attributes meta-box.

[kallehauge]

Is your feature request related to a problem? Please describe.
It is not a given what Default template is for users of a WordPress website with themes that use multiple types of templates for layout.
So since WP version 4.1.0 there have been a filter to change the name that appears in the page-attributes meta box:

apply_filters( 'default_page_template_title', __( 'Default Template' ), 'meta-box' );
apply_filters( 'default_page_template_title', __( 'Default Template' ), 'quick-edit' );

This filter is currently not supported with Gutenberg since it is hardcoded in editor/components/page-attributes/template.js (at the time of writing it's on line 34).

Describe the solution you'd like
I would like to be able to change the name of the default template (aka when there is no value). This could either be done with the existing default_page_template_title hook or a new solution.

Describe alternatives you've considered
I have looked into how the templates are currently being fetched and this is being done with wp_get_theme()->get_page_templates() in gutenberg_editor_scripts_and_styles().
An alternative to this solution would be to wrap the call in a PHP function and set all the values inside the PHP function and use the default_page_template_title filter for the default value.