This lets us avoid each UI needing to handle link clicks directly, and
lets actions stored in LibWebView avoid awkwardly going through the link
click callbacks to open URLs.
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.
Initializing m_current_screen may not be necessary, but the initial
value of m_preferred_color_scheme is sent to WebContent, so failing to
give it an initializer would cause bugs.
