citra_qt: Use the new verify backend; UI changes

Displayed username along with nickname (when they are not identical); Requested and displayed user's avatar; Made the dialog bigger for extended names.

Added a few functions to web_backend (GetImage, GetPlain) to support getting data in multiple content-types.

Added a no_avatar icon for users without avatars.

dist/qt_themes/default/default.qrc

@@ -18,6 +18,8 @@
 
         <file alias="48x48/folder.png">icons/48x48/folder.png</file>
 
+        <file alias="48x48/no_avatar.png">icons/48x48/no_avatar.png</file>
+
         <file alias="48x48/plus.png">icons/48x48/plus.png</file>
       
         <file alias="48x48/sd_card.png">icons/48x48/sd_card.png</file>