Simplification: get rid of objecthandle reuse across sessions

It isn't used, and doesn't make much sense. The only thing it does is
create a risk that objecthandles will eventually wrap around.
Refactor: modernize enumerateStorages()

New-style signals, Qt-style indentation
bugfix in enumerateStorages: get largest puoids from all storages

With multiple storages involved, it's important to get the largest
puoid of all before starting enumeration in any of them.
Refactor: nicer syntax for comparing MtpInt128 objects
refactor: modernize initStorages()
storagefactory emits storageReady() when all plugins are ready

This a refactoring in preparation for doing the storage enumeration
asynchronously. In this commit the signal is hooked up but nothing
is done with it yet.
cleanup: fix compiler warning
[mtp] Do the file scan in the background

Some initiators (OS/X) didn't like that we took time for the file scan
while handling the GetStorageIDs request. Work around it by doing the
scan asynchronously while the protocol starts up, then only delaying
a request if the results of the scan are actually needed.