QR code fixes

New classes ScanQRTextEdit and ShowQRTextEdit.
Reason: dependencies on zbar availability and issues with the QRTextEdit constructor.
- ScanQRTextEdit needs access to the config (fetch camera). It needs to load
the zbar processor properly before trying to scan. Keeping a reference to
the processor in qrscaner fixes the crashes on windows.
- ShowQRTextEdit should not have access to scan_qr().
- no need to setReadOnly anymore. It is clear from the class name.

Show master pub keys now has a Combobox if multiple accounts are
available.

gui/qt/installwizard.py

@@ -128,7 +128,7 @@ class InstallWizard(QDialog):
     def enter_seed_dialog(self, msg, sid, func=None):
         if func is None:
             func = self.is_any
-        vbox, seed_e = seed_dialog.enter_seed_box(msg, sid)
+        vbox, seed_e = seed_dialog.enter_seed_box(msg, self, sid)
         vbox.addStretch(1)
         hbox, button = ok_cancel_buttons2(self, _('Next'))
         vbox.addLayout(hbox)