This introduces a simple FileDownloader to download files in the UI
process from RequestServer. We use this to download the context menu
image - this download is likely to hit the disk cache.
By migrating the debug menu to LibWebView, the AppKit and Qt UIs are now
in sync - the AppKit UI was previously missing some actions.
Further, this inadvertently fixes bugs around applying debug settings to
new web views, especially across site-isolated processes. We were
previously not applying settings appropriately; this now "just works" in
the LibWebView infra.
This migrates all duplicated context menus from the UIs to LibWebView.
The context menu actions are now largely handled directly in LibWebView,
with some UI-specific callbacks added to display e.g. confirmation
dialogs.
Actions that only ever apply to a specific web view are stored on the
ViewImplementation itself. Actions that need to be dynamically applied
to the active web view are stored on the Application.
We currently duplicate a lot of code to handle application/context menus
and actions. The goal here is to hold the data for the menus and actions
in LibWebView. Each UI will then be able to generate menus from the data
on-the-fly.
The structures added here are meant to support generic and checkable
actions, action groups, submenus, etc.
