Better install wizard

Break out the workflow logic of the install wizard
into a base class.  This means reimplementing with
full support in a new GUI is now easy; you just provide
ways to request passwords, show messages etc.  The API
is fully documented in the base class.

There are a couple of minor outstanding issues, including that
the old messages shown when recovering a wallet are missing.
I will come back to that.  Ledger wallet might be broken.

Other improvements:

The install wizard code is now easy to follow and understand.
Hardware wallets can now be restored without any need for their
accompanying libraries.
Various bits of trustedcoin were broken and have been fixed.
Many plugin hooks can be removed.  I have only started on this.

plugins/trezor/trezor.py

@@ -1,12 +1,6 @@
 from plugins.trezor.client import trezor_client_class
 from plugins.trezor.plugin import TrezorCompatiblePlugin, TrezorCompatibleWallet
 
-try:
-    from trezorlib.client import proto, BaseClient, ProtocolMixin
-    TREZOR = True
-except ImportError:
-    TREZOR = False
-
 
 class TrezorWallet(TrezorCompatibleWallet):
     wallet_type = 'trezor'
@@ -14,12 +8,16 @@ class TrezorWallet(TrezorCompatibleWallet):
 
 
 class TrezorPlugin(TrezorCompatiblePlugin):
-    client_class = trezor_client_class(ProtocolMixin, BaseClient, proto)
     firmware_URL = 'https://www.mytrezor.com'
     libraries_URL = 'https://github.com/trezor/python-trezor'
-    libraries_available = TREZOR
     minimum_firmware = (1, 2, 1)
     wallet_class = TrezorWallet
-    import trezorlib.ckd_public as ckd_public
-    from trezorlib.client import types
-    from trezorlib.transport_hid import HidTransport
+    try:
+        from trezorlib.client import proto, BaseClient, ProtocolMixin
+        client_class = trezor_client_class(ProtocolMixin, BaseClient, proto)
+        import trezorlib.ckd_public as ckd_public
+        from trezorlib.client import types
+        from trezorlib.transport_hid import HidTransport
+        libraries_available = True
+    except ImportError:
+        libraries_available = False