Read backend after args.temp_settings

This should hopefully not affect any config change handlers, as almost nothing
is registered this early.

Fixes #3340

doc/changelog.asciidoc

@@ -104,6 +104,7 @@ Fixed
 - Fixed crash when opening `qute://help/img`
 - Fixed `gU` (`:navigate up`) on `qute://help` and webservers not handling `..`
   in a URL.
+- Using e.g. `-s backend webkit` to set the backend now works correctly.
 
 Deprecated
 ~~~~~~~~~~

qutebrowser/config/configinit.py

@@ -66,14 +66,14 @@ def early_init(args):
 
     configfiles.init()
 
-    objects.backend = get_backend(args)
-
     for opt, val in args.temp_settings:
         try:
             config.instance.set_str(opt, val)
         except configexc.Error as e:
             message.error("set: {} - {}".format(e.__class__.__name__, e))
 
+    objects.backend = get_backend(args)
+
     configtypes.Font.monospace_fonts = config.val.fonts.monospace
     config.instance.changed.connect(_update_monospace_fonts)