Use QDesktopServices to open cover images instead of webbrowser2

QDesktopServices takes care of honoring the user configuration so
when opening an image, the user preferred external app is used
instead of always using a web browser.

Fixes PICARD-1501

picard/ui/infodialog.py

@@ -38,8 +38,7 @@ from picard.util import (
     encode_filename,
     format_time,
     htmlescape,
-    union_sorted_lists,
-    webbrowser2,
+    union_sorted_lists
 )
 
 from picard.ui import PicardDialog
@@ -243,7 +242,8 @@ class InfoDialog(PicardDialog):
             return
         filename = data.tempfile_filename
         if filename:
-            webbrowser2.open("file://" + filename)
+            url = QtCore.QUrl.fromLocalFile(filename)
+            QtGui.QDesktopServices.openUrl(url)
 
 
 def format_file_info(file_):