This refactoring is driven by the need to be able to do true queued RPC calls
(where each successive call uses the result of the previous).
Currently, such queueing of requests is done by assigning them special "magic"
tag numbers, which are then intercepted in one big switch() statement and acted
upon. This (aside from making code greatly unclear) effectively makes each such
queue a singleton, because state passing is restricted to global variables.
We refactor RpcClient to assign an unique tag to each remote call, and then
abstract all the call<->response matching with Qt's future/promise mechanism.
Finally, we introduce a "RPC request queue" class (RpcQueue) which is built on
top of QFutureWatcher and C++11's <functional> library. This class maintains
a queue of functions, where each function receives an RPC response, does
necessary processing, performs another call and finally returns its future.
This has a couple of benefits: 1) it is clearly visible to the user that
the list is filtered (doesn't display all the torrents) even when filter
bar is hidden, 2) doesn't lead to filter bar controls being shifted to
the left/right as when "Show:" label text changes.
Disable "Open Folder" and "Verify Local Data" actions for magnets in
Torrent menu. If more than one torrent is selected, "Verify Local Data"
is enabled but only non-magnets are verified if activated.
Add a few sanity checks when opening folder, just in case.
Split FileTree.{h,cc} and FilterBar.{h,cc} files so that each class
is in its own file.
This breaks translations (some classes got renamed => context changed),
to be fixed by next commit (along with Tx sync).
