greasemonkey: enable running in isolated js worlds

QtWebEngine (via chromium) has the ability to run injected scripts in
isolated "worlds". What is isolated is just the javascript environment,
so variables and functions defined by the page and the script won't
clobber each other, or be able to interact (including variables saved to
the global `window` object). The DOM is still accessible from "isolated"
scripts.

This is NOT a security measure. You cannot put untrusted scripts in one
of these isolated worlds and expect it to not be able to do whatever
page js can do, it is just for namespacing convenience. See
https://stackoverflow.com/questions/9515704/insert-code-into-the-page-context-using-a-content-script
for some examples of how to inject scripts into the page scope using DOM
elements.

Now you can specify the world ID in a `@qute-js-world` directive like:

```
// ==UserScript==
// @name         Do thing
// @match        *://some.site/*
// @qute-js-world 1234
// ==/UserScript==
document.body.innerHTML = "<strong>overwritten</strong>"
```

The QtWebEngine docs say worldid is a `quint32` so you can put whatever
number (positive, whole, real) you want there. I have chosen to allow
the `qutebrowser.utils.usertypes` enum as aliases for IDs that are
predefined in
`qutebrowser.browser.webengine.webenginetab._JS_WORLD_MAP`. So you can
pass `main`, `application`, `user` or `jseval` in there too. `main` (0)
is the default one and is the only one in which JS disabled when
`content.javascript.enabled` is set to `false`. All others are still
enabled.

I'm not sure whether using any of those already-named worlds makes
sense, apart from `main`. We could stop people from using them I
suppose. Another option is to allow people to pass in `*` as a value to
have scripts put into their own little worlds, probably backed by a
counter in the GreaseMonkeyManager class.

Chrome docs: https://developer.chrome.com/extensions/content_scripts#execution-environment
Webengine docs: https://doc.qt.io/qt-5/qwebenginescript.html#details

qutebrowser/browser/greasemonkey.py

@@ -58,6 +58,7 @@ class GreasemonkeyScript:
         self.run_at = None
         self.script_meta = None
         self.runs_on_sub_frames = True
+        self.jsworld = "main"
         for name, value in properties:
             if name == 'name':
                 self.name = value
@@ -77,6 +78,8 @@ class GreasemonkeyScript:
                 self.runs_on_sub_frames = False
             elif name == 'require':
                 self.requires.append(value)
+            elif name == 'qute-js-world':
+                self.jsworld = value
 
     HEADER_REGEX = r'// ==UserScript==|\n+// ==/UserScript==\n'
     PROPS_REGEX = r'// @(?P<prop>[^\s]+)\s*(?P<val>.*)'

qutebrowser/browser/webengine/webenginetab.py

@@ -900,7 +900,18 @@ class _WebEngineScripts(QObject):
             # @run-at (and @include/@exclude/@match) is parsed by
             # QWebEngineScript.
             new_script = QWebEngineScript()
-            new_script.setWorldId(QWebEngineScript.MainWorld)
+            try:
+                world = int(script.jsworld)
+            except ValueError:
+                try:
+                    world = _JS_WORLD_MAP[usertypes.JsWorld[
+                        script.jsworld.lower()]]
+                except KeyError:
+                    log.greasemonkey.error(
+                        "script {} has invalid value for '@qute-js-world'"
+                        ": {}".format(script.name, script.jsworld))
+                    continue
+            new_script.setWorldId(world)
             new_script.setSourceCode(script.code())
             new_script.setName("GM-{}".format(script.name))
             new_script.setRunsOnSubFrames(script.runs_on_sub_frames)