Improve the Boost Union settings navigation

[abias]

During the work on the solution for #482 which happened in #609 and #612 , it became clear that the Boost Union settings navigation is not as usable and intuitive as it should be.

Much of the current status is caused by the circumstances of how Moodle core handles settings. However, these discussion items have been raised and should be considered:

• The breadcrumbs on the settings pages should link to the settings overview page and not to the settings category
• It would be really helpful if you could jump from one settings page of Boost Union (e.g. 'Look') to another one (e.g. 'Feel') directly without going through the whole navigation tree again (which is mainly caused by the fact that the breadcrumb is not helpful in Boost Union settings).
• It has to be discussed if, after the settings overview page has been introduced, the direct links to all the settings pages on /admin/search.php#linkappearance are still helpful or rather misleading.

[wiebkemueller-hsh]

In the Boost Union support group we have discussed the idea of inserting another menu (or tabs) in the intermediate level in addition to a more helpful breadcrumb. This briefly sketched scribble on a screenshot hopefully visualizes the ideas we had.

Any comments on this or further ideas from the community are welcome!