Set the default zoom initially with QtWebEngine

This solves a part of #1993, however when the user navigates to a
different page, the zoom is reset.

qutebrowser/browser/browsertab.py

@@ -570,7 +570,9 @@ class AbstractTab(QWidget):
         self.search._widget = widget
         self.printing._widget = widget
         self.elements._widget = widget
+
         self._install_event_filter()
+        self.zoom.set_default()
 
     def _install_event_filter(self):
         raise NotImplementedError

qutebrowser/browser/webkit/webkittab.py

@@ -605,7 +605,6 @@ class WebKitTab(browsertab.AbstractTab):
         self.elements = WebKitElements(self)
         self._set_widget(widget)
         self._connect_signals()
-        self.zoom.set_default()
         self.backend = usertypes.Backend.QtWebKit
 
     def _install_event_filter(self):

tests/end2end/features/zoom.feature

@@ -85,3 +85,9 @@ Feature: Zooming in and out
         And I run :zoom-in
         Then the message "Zoom level: 120%" should be shown
         And the zoom should be 120%
+
+    Scenario: Setting a default zoom
+        When I set ui -> default-zoom to 200%
+        And I open data/hello.txt in a new tab
+        And I run :tab-only
+        Then the zoom should be 200%