Only clear favicons on load with QtWebKit

QtWebEngine seems to automatically clear the favicon when loading e.g.
about:blank, and not clearing it there again fixes #3469.
Original issue: #187

doc/changelog.asciidoc

@@ -102,6 +102,7 @@ Fixed
 - QtWebEngine: Keys like `Ctrl-V` or `Shift-Insert` are now correctly
   handled/filtered with Qt 5.10.
 - QtWebEngine: Fixed hangs/segfaults on exit with Qt 5.10.1.
+- QtWebEngine: Fixed favicons sometimes getting cleared with Qt 5.10.
 - QtWebKit: `:view-source` now displays a valid URL.
 - URLs containing ampersands and other special chars are now shown
   correctly when filtering them in the completion.

qutebrowser/browser/webkit/webkittab.py

@@ -30,7 +30,7 @@ import pygments.formatters
 import sip
 from PyQt5.QtCore import (pyqtSlot, Qt, QEvent, QUrl, QPoint, QTimer, QSizeF,
                           QSize)
-from PyQt5.QtGui import QKeyEvent
+from PyQt5.QtGui import QKeyEvent, QIcon
 from PyQt5.QtWebKitWidgets import QWebPage, QWebFrame
 from PyQt5.QtWebKit import QWebSettings
 from PyQt5.QtPrintSupport import QPrinter
@@ -743,6 +743,8 @@ class WebKitTab(browsertab.AbstractTab):
     def _on_load_started(self):
         super()._on_load_started()
         self.networkaccessmanager().netrc_used = False
+        # Make sure the icon is cleared when navigating to a page without one.
+        self.icon_changed.emit(QIcon())
 
     @pyqtSlot()
     def _on_frame_load_finished(self):

qutebrowser/mainwindow/tabbedbrowser.py

@@ -552,7 +552,6 @@ class TabbedBrowser(tabwidget.TabWidget):
         if tab.data.keep_icon:
             tab.data.keep_icon = False
         else:
-            self.setTabIcon(idx, QIcon())
             if (config.val.tabs.tabs_are_windows and
                     config.val.tabs.favicons.show):
                 self.window().setWindowIcon(self.default_window_icon)