Better support for USB devices

Benefits of this rewrite include:

- support of disconnecting / reconnecting a device without having
  to close the wallet, even in a different USB socket
- support of multiple keepkey / trezor devices, both during wallet
  creation and general use
- wallet is watching-only dynamically according to whether the
  associated device is currently plugged in or not

.gitignore

@@ -1,6 +1,4 @@
 ####-*.patch
-gui/icons_rc.py
-lib/icons_rc.py
 *.pyc
 *.swp
 build/