Implement MouseLock permission

Closes #3287

doc/changelog.asciidoc

@@ -24,6 +24,7 @@ Added
 - The qute-pass userscript now has optional OTP support.
 - When `:spawn --userscript` is called with a count, that count is now
   passed to userscripts as `$QUTE_COUNT`.
+- New `content.mouse_lock` setting to handle HTML5 pointer locking.
 
 Changed
 ~~~~~~~

qutebrowser/browser/webengine/webenginetab.py

@@ -707,6 +707,18 @@ class _WebEnginePermissions(QObject):
             QWebEnginePage.MediaVideoCapture: 'record video',
             QWebEnginePage.MediaAudioVideoCapture: 'record audio/video',
         }
+        try:
+            options.update({
+                QWebEnginePage.MouseLock:
+                    'content.mouse_lock',
+            })
+            messages.update({
+                QWebEnginePage.MouseLock:
+                    'hide your mouse pointer',
+            })
+        except AttributeError:
+            # Added in Qt 5.8
+            pass
         try:
             options.update({
                 QWebEnginePage.DesktopVideoCapture:

qutebrowser/config/configdata.yml

@@ -354,6 +354,15 @@ content.geolocation:
   supports_pattern: true
   desc: Allow websites to request geolocations.
 
+content.mouse_lock:
+  default: ask
+  type: BoolAsk
+  supports_pattern: true
+  backend:
+    QtWebKit: false
+    QtWebEngine: Qt 5.8
+  desc: Allow websites to lock your mouse pointer.
+
 content.headers.accept_language:
   type:
     name: String