Add proxy support for QtWebEngine and Qt 5.7.1

This used to give us crashes in libproxy:
https://github.com/libproxy/libproxy/issues/45
https://bugreports.qt.io/browse/QTBUG-56852

However, trying again with Qt 5.7.1 on Debian and from PyPI, this doesn't happen
anymore, so it was probably something with how Archlinux handled things.

See #2082, #2775.
Reverts fd29528e4f

CHANGELOG.asciidoc

@@ -45,6 +45,7 @@ Added
   customize statusbar colors for private windows.
 - New `{private}` field displaying `[Private Mode]` for
   `ui -> window-title-format` and `tabs -> title-format`.
+- (QtWebEngine) Proxy support with Qt 5.7.1 too
 
 Changed
 ~~~~~~~

doc/help/settings.asciidoc

@@ -789,8 +789,6 @@ The proxy to use.
 
 In addition to the listed values, you can use a `socks://...` or `http://...` URL.
 
-This setting only works with Qt 5.8 or newer when using the QtWebEngine backend.
-
 Valid values:
 
  * +system+: Use the system wide proxy.

qutebrowser/app.py

@@ -410,10 +410,8 @@ def _init_modules(args, crash_handler):
     log.init.debug("Initializing network...")
     networkmanager.init()
 
-    if qtutils.version_check('5.8'):
-        # Otherwise we can only initialize it for QtWebKit because of crashes
-        log.init.debug("Initializing proxy...")
-        proxy.init()
+    log.init.debug("Initializing proxy...")
+    proxy.init()
 
     log.init.debug("Initializing readline-bridge...")
     readline_bridge = readline.ReadlineBridge()

qutebrowser/browser/webkit/webkittab.py

@@ -33,7 +33,6 @@ from PyQt5.QtWebKit import QWebSettings
 from PyQt5.QtPrintSupport import QPrinter
 
 from qutebrowser.browser import browsertab
-from qutebrowser.browser.network import proxy
 from qutebrowser.browser.webkit import webview, tabhistory, webkitelem
 from qutebrowser.browser.webkit.network import webkitqutescheme
 from qutebrowser.utils import qtutils, objreg, usertypes, utils, log, debug
@@ -42,12 +41,6 @@ from qutebrowser.utils import qtutils, objreg, usertypes, utils, log, debug
 def init():
     """Initialize QtWebKit-specific modules."""
     qapp = QApplication.instance()
-
-    if not qtutils.version_check('5.8'):
-        # Otherwise we initialize it globally in app.py
-        log.init.debug("Initializing proxy...")
-        proxy.init()
-
     log.init.debug("Initializing js-bridge...")
     js_bridge = webkitqutescheme.JSBridge(qapp)
     objreg.register('js-bridge', js_bridge)

qutebrowser/config/configdata.py

@@ -437,14 +437,10 @@ def data(readonly=False):
              "User agent to send. Empty to send the default."),
 
             ('proxy',
-             SettingValue(typ.Proxy(), 'system',
-                          backends=(None if qtutils.version_check('5.8')
-                                    else [usertypes.Backend.QtWebKit])),
+             SettingValue(typ.Proxy(), 'system'),
              "The proxy to use.\n\n"
              "In addition to the listed values, you can use a `socks://...` "
-             "or `http://...` URL.\n\n"
-             "This setting only works with Qt 5.8 or newer when using the "
-             "QtWebEngine backend."),
+             "or `http://...` URL."),
 
             ('proxy-dns-requests',
              SettingValue(typ.Bool(), 'true',

scripts/dev/src2asciidoc.py

@@ -365,8 +365,7 @@ 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')]
+    version_dependent_options = [('general', 'print-element-backgrounds')]
     for optname, option in sect.items():
         f.write("\n")
         f.write('[[{}-{}]]'.format(sectname, optname) + "\n")