Add settings to block various types of requests

Note: If you're looking at this as a first contribution, please read https://github.com/qutebrowser/qutebrowser/issues/2626#issuecomment-754531629 - some information in this comment is likely outdated (or makes this more complicated that it needs to be in a first step).

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This is sort of the first step torwards uMatrix-like functionality: Having a way to block various requests based from data in QWebEngineUrlRequestInfo (per-domain after #27 of course, which is pretty much uMatrix minus the UI).

The main interesting thing there is ResourceType, with the following types:

• MainFrame (Top level page)
  • Probably no point in allowing to block this
• SubFrame (Frame or iframe)
  • Should be blockable
  • Blockable in uMatrix
  • Might be blockable in QtWebKit too via acceptNavigationRequest
• Stylesheet (A CSS stylesheet)
  • Should be blockable
  • Blockable in uMatrix
  • Probably not blockable in QtWebKit (maybe via mimetype?)
• Script (An external script)
  • Already handled via content -> allow-javascript.
  • If they still occur with the setting off, might be good to block them
  • Blockable in uMatrix
• Image (An image (JPG, GIF, PNG, and so on))
  • Already handled via content -> allow-images.
  • If they still occur with the setting off, might be good to block them
  • Blockable in uMatrix
• FontResource (A font)
  • Should be blockable if those are web fonts
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit (maybe via mimetype?)
• SubResource (An "other" subresource)
  • Unclear when it happens... Probably fine to leave it right now.
• Object (An object (or embed) tag for a plugin or a resource that a plugin requested)
  • Should maybe be blockable?
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit (maybe via mimetype?)
• Media (A media resource)
  • Should be blockable
  • Blockable in uMatrix
  • Maybe blockable in QtWebKit via mimetype?
  • QtWebKit-NG has QWebSettings::MediaEnabled - we should be able to use the int value without PyQt support
  • Doesn't seem to block YouTube though, as that uses XHR requests...
  • Issue for click-to-play in QtWebEngine: QTBUG-60799
  • See https://github.com/annulen/webkit/issues/357 for click-to-play on QtWebKit; QtWebEngine still needs an issue for this
• Worker (The main resource of a dedicated worker)
  • Should maybe be blockable?
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit
• SharedWorker (The main resource of a shared worker)
  • Should maybe be blockable?
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit
• Prefetch (An explicitly requested prefetch)
  • Unclear when it happens...
  • Maybe this would allow to implement network -> dns-prefetch for QtWebEngine?
  • Not blockable in uMatrix
• Favicon
  • Already handled via content -> allow-images.
  • If they still occur with the setting off, might be good to block them
  • Not blockable in uMatrix
• Xhr (An XMLHttpRequest)
  • Should be blockable
  • Blockable in uMatrix
  • Probably not blockable in QtWebKit
• Ping (A ping request for )
  • Already handled via content -> hyperlink-auditing.
  • If they still occur with the setting off, might be good to block them
  • Blockable in uMatrix
• ServiceWorker (The main resource of a service worker)
  • Should maybe be blockable?
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit
• CspReport (A report of Content Security Policy (CSP) violations)
  • Should maybe be blockable?
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit
• PluginResource (A resource requested by a plugin)
  • Should maybe be blockable?
  • Not blockable in uMatrix
  • Probably not blockable in QtWebKit
• Unknown / Other
  • Should be blockable
  • Blockable in uMatrix
  • Probably not blockable in QtWebKit