Add support for general -> print-element-backgrounds with WebEngine

CHANGELOG.asciidoc

@@ -29,6 +29,7 @@ Added
 - Support for the `storage -> cache-size` option with QtWebEngine
 - Support for the `colors -> webpage.bg` option with QtWebEngine
 - Support for the HTML5 fullscreen API (e.g. youtube videos) with QtWebEngine
+- Support for the `general -> print-element-backgrounds` option with QtWebEngine on Qt >= 5.8
 
 Changed
 ~~~~~~~

qutebrowser/browser/webengine/webenginesettings.py

@@ -185,7 +185,6 @@ def shutdown():
 # - AutoLoadIconsForPage (5.7)
 # - TouchIconsEnabled (5.7)
 # - FocusOnNavigationEnabled (5.8)
-# - PrintElementBackgrounds (5.8)
 # - AllowRunningInsecureContent (5.8)
 #
 # Missing QtWebEngine fonts:
@@ -288,3 +287,10 @@ try:
 except AttributeError:
     # Added in Qt 5.7
     pass
+
+try:
+    MAPPINGS['general']['print-element-backgrounds'] = Attribute(
+        QWebEngineSettings.PrintElementBackgrounds)
+except AttributeError:
+    # Added in Qt 5.8
+    pass

qutebrowser/config/configdata.py

@@ -200,9 +200,12 @@ def data(readonly=False):
 
             ('print-element-backgrounds',
              SettingValue(typ.Bool(), 'true',
-                          backends=[usertypes.Backend.QtWebKit]),
+                          backends=(None if qtutils.version_check('5.8')
+                                    else [usertypes.Backend.QtWebKit])),
              "Whether the background color and images are also drawn when the "
-             "page is printed."),
+             "page is printed.\n"
+             "This setting only works with Qt 5.8 or newer when using the "
+             "QtWebEngine backend."),
 
             ('xss-auditing',
              SettingValue(typ.Bool(), 'false'),

scripts/dev/src2asciidoc.py

@@ -365,6 +365,8 @@ def generate_commands(filename):
 
 def _generate_setting_section(f, sectname, sect):
     """Generate documentation for a single section."""
+    version_dependent_options = [('network', 'proxy'),
+                                 ('general', 'print-element-backgrounds')]
     for optname, option in sect.items():
         f.write("\n")
         f.write('[[{}-{}]]'.format(sectname, optname) + "\n")
@@ -391,7 +393,7 @@ def _generate_setting_section(f, sectname, sect):
             f.write("Default: empty\n")
 
         if (option.backends is None or
-                (sectname, optname) == ('network', 'proxy')):
+                (sectname, optname) in version_dependent_options):
             pass
         elif option.backends == [usertypes.Backend.QtWebKit]:
             f.write("\nThis setting is only available with the QtWebKit "