[fix] don't show a category if there is no active engine in

When deactivate all the engines of a category, this category should disappeare.
This feature has been lost in commit 8e9ad1cc.

For better readability, webapp.get_enabled_categories() has been rewritten with
identical functionality.

Related:

- https://github.com/searxng/searxng/issues/1020
- https://github.com/searxng/searxng/issues/1604



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>