Improvements for GreaseMonkey support #3238

Things which could be improved after #3040 is in:

• Maybe offer an install dialog when a .user.js file is opened, like Chromium
• Have some kind of updater for userscripts?
• Support @require (maybe without the downloading, but at least load those files if they exist?)
• Try stubbing exportFunction and maybe set a more correct scriptHandler and version then for GM_info?
• Support isPrivate in GM_info.
• Look into GreaseMonkey's sandboxing/security features
• Set up a QtWebChannel so we can give scripts elevated permissions
  • Use some kind of database for GM_getValue et al so it's isolated from the page's localStorage
  • Use a QNetworkAccessManager for GM_xmlhttpRequest so it can be cross-origin.
    • Looks like there's some WIP work
    • But apparently that sometimes hangs?
  • Maybe also something for GM_openInTab
• Support for the GreaseMonkey 4 API.
• Investigate why @noframes sometimes doesn't work with QtWebEngine / QWebEngineScript
• Support GM_registerMenuCommand somehow
• Can we disable the page's JavaScript exception handler while running the Greasemonkey script?
• Can we set the injection point correctly for a Qt 5.8+ QtWebEngine? (my bad, Qt already does this based on the userscript comment)
• Maybe a missing run-at shouldn't show an error?
• Regex for include/exclude
• Use UrlPattern from Implement URL match patterns #3369 for @match
• Support a custom metadata directive like @qute-js-world to request the script be ran in a non-default javascript "world"
• Alias/map qute-js-world to @inject-into https://violentmonkey.github.io/posts/inject-into-context/
• Implement GM_getResourceText (Wanikani Niai Similar Kanji called unimplemented GM_getResourceText #3975)
• Require @grant unsafeWindow to use unsafeWindow
• Somehow fix up line numbers, e.g. via source maps? (Correct line number for greasemonkey user script #5080)
• Accept relative @requires directives, without requiring the use of the file:// protocol (from Greasemonkey scripts: Local file improvements #6458)
• Load local files directly rather than needing to copy them (from Greasemonkey scripts: Local file improvements #6458)
• Support GM_setValue with non-primitive values (see Improve some Greasemonkey API stubs #6699)
• Update docs and commands to not seem so Greasemonkey (the extension) specific (from Violentmonkey support #7286)
• Add some Greasemonkey docs for qutebrowser, also e.g. @qute-js-world

cc @toofar