globally search for media
this pr allows passing 0 to dc_get_chat_media(), the function returns media from any chat then - similar to dc_search_msgs().
moreover, this pr fixes a minor issue to not return hidden media - and a test (dc_get_chat_media() was completely untested)
closes #3527
one thing @adbenitez pointed out in dev chat, is that this approach does not care for duplicates - in a selector you maybe do not want to have them. however, unless there is a dead-simple approach, to allow UI to move forward, it probably makes sense to care for that at some later point in a subsequent pr. also, there is the rough idea of files deduplication, this may help somehow as well ;)
rust fmt is unhappy now :)
EDIT: pushed a fix.
k, then let's merge that in to go forward with https://github.com/deltachat/deltachat-ios/pull/1668 :)