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.

lib/qrscanner.py

@@ -6,28 +6,37 @@ try:
 except ImportError:
     zbar = None
 
+proc = None
 
-def scan_qr(config):
+def init(config):
     if not zbar:
         raise BaseException("\n".join([_("Cannot start QR scanner."),_("The zbar package is not available."),_("On Linux, try 'sudo apt-get install python-zbar'")]))
     device = config.get("video_device", "default")
     if device == 'default':
         device = ''
+    global proc
     proc = zbar.Processor()
     proc.init(video_device=device)
+
+def scan_qr(self):
+    if not zbar:
+        raise BaseException("\n".join([_("Cannot start QR scanner."),_("The zbar package is not available."),_("On Linux, try 'sudo apt-get install python-zbar'")]))
+    if proc is None:
+        raise BaseException("Start proc first")
     proc.visible = True
     while True:
         try:
             proc.process_one()
         except Exception:
             # User closed the preview window
-            return {}
+            return ""
         for r in proc.results:
             if str(r.type) != 'QRCODE':
                 continue
+            # hiding the preview window stops the camera
+            proc.visible = False
             return r.data
 
-
 def _find_system_cameras():
     device_root = "/sys/class/video4linux"
     devices = {} # Name -> device