Improvement: Maybe the categorybreadcrumbs setting could use less dupliated code

[abias]

This is a follow-up issue to #317 / #483:

@danowar2k commented in #317:

This now leads to me wondering whether it would be better to fix the wrong order of section > category or to first discuss why the setting categorybreadcrumbs exists while enabling navshowcategories and navshowmycoursecategories also has the desired effect of adding categories to the course breadcrumb (meaning I don't get what's missing there that Union fixes with its setting).

The Boost Union setting categorybreadcrumbs is there to show a breadcrumb on the course overview pages. This is something which the Moodle core does not do in any way anymore, even not if navshowcategories or navshowmycoursecategories is enabled.

But basically, @danowar2k is right:
To realize the Boost Union setting categorybreadcrumbs, a large amount of code has been duplicated into Boost Union - the /classes/boostnavbar.php class file has been duplicated and the get_course_categories() function from lib/navigation.lib has been added to this file.

It would be worthwhile to investigate if at least a part of the duplicated code can be refactored to use Moodle core functions instead to avoid issues like #317 in the future.