Allow to set cookies-store at runtime with Qt 5.9

Fixes #2588

CHANGELOG.asciidoc

@@ -50,6 +50,8 @@ Changed
 - When ui -> message-timeout is set to 0, messages are now never cleared.
 - Middle/right-clicking the blank parts of the tab bar (when vertical) now
   closes the current tab.
+- With Qt 5.9, `content -> cookies-store` can now be set on QtWebEngine without
+  a restart.
 
 Fixed
 ~~~~~

doc/help/settings.asciidoc

@@ -175,7 +175,7 @@
 |<<content-local-content-can-access-remote-urls,local-content-can-access-remote-urls>>|Whether locally loaded documents are allowed to access remote urls.
 |<<content-local-content-can-access-file-urls,local-content-can-access-file-urls>>|Whether locally loaded documents are allowed to access other local urls.
 |<<content-cookies-accept,cookies-accept>>|Control which cookies to accept.
-|<<content-cookies-store,cookies-store>>|Whether to store cookies. Note this option needs a restart with QtWebEngine.
+|<<content-cookies-store,cookies-store>>|Whether to store cookies. Note this option needs a restart with QtWebEngine on Qt < 5.9.
 |<<content-host-block-lists,host-block-lists>>|List of URLs of lists which contain hosts to block.
 |<<content-host-blocking-enabled,host-blocking-enabled>>|Whether host blocking is enabled.
 |<<content-host-blocking-whitelist,host-blocking-whitelist>>|List of domains that should always be loaded, despite being ad-blocked.
@@ -1616,7 +1616,7 @@ This setting is only available with the QtWebKit backend.
 
 [[content-cookies-store]]
 === cookies-store
-Whether to store cookies. Note this option needs a restart with QtWebEngine.
+Whether to store cookies. Note this option needs a restart with QtWebEngine on Qt < 5.9.
 
 Valid values:
 

qutebrowser/browser/webengine/webenginesettings.py

@@ -34,7 +34,8 @@ from PyQt5.QtWebEngineWidgets import (QWebEngineSettings, QWebEngineProfile,
 
 from qutebrowser.browser import shared
 from qutebrowser.config import config, websettings
-from qutebrowser.utils import objreg, utils, standarddir, javascript, log
+from qutebrowser.utils import (objreg, utils, standarddir, javascript, log,
+                               qtutils)
 
 
 class Attribute(websettings.Attribute):
@@ -177,7 +178,8 @@ def init(args):
     _init_stylesheet(profile)
     # We need to do this here as a WORKAROUND for
     # https://bugreports.qt.io/browse/QTBUG-58650
-    PersistentCookiePolicy().set(config.get('content', 'cookies-store'))
+    if not qtutils.version_check('5.9'):
+        PersistentCookiePolicy().set(config.get('content', 'cookies-store'))
 
     Attribute(QWebEngineSettings.FullScreenSupportEnabled).set(True)
 
@@ -221,9 +223,6 @@ MAPPINGS = {
             Attribute(QWebEngineSettings.LocalContentCanAccessRemoteUrls),
         'local-content-can-access-file-urls':
             Attribute(QWebEngineSettings.LocalContentCanAccessFileUrls),
-        # https://bugreports.qt.io/browse/QTBUG-58650
-        # 'cookies-store':
-        #     PersistentCookiePolicy(),
         'webgl':
             Attribute(QWebEngineSettings.WebGLEnabled),
     },
@@ -301,3 +300,8 @@ try:
 except AttributeError:
     # Added in Qt 5.8
     pass
+
+
+if qtutils.version_check('5.9'):
+    # https://bugreports.qt.io/browse/QTBUG-58650
+    MAPPINGS['content']['cookies-store'] = PersistentCookiePolicy()

qutebrowser/config/configdata.py

@@ -915,7 +915,7 @@ def data(readonly=False):
             ('cookies-store',
              SettingValue(typ.Bool(), 'true'),
              "Whether to store cookies. Note this option needs a restart with "
-             "QtWebEngine."),
+             "QtWebEngine on Qt < 5.9."),
 
             ('host-block-lists',
              SettingValue(