mtp: use LIBMTP_Get_Children to implement async listing

LIBMTP_Get_Children returns a list of object ids within a folder, which
returns loads faster than the existing files listing API. once we have
the list of ids we can asynchronously resolve the actual file entries
for them and emit them as they appear. this allows us to enumerate
entries as they appear rather than all at once.

to facilitate this behavior a new function getFilesAndFolders2 was
introduced that returns a qdbusobjectpath. under this path a new
MTPLister object is registered that emits the listing.

CCBUG: 462059