When a window containing a WebView becomes visibile, we have to inform
WebContent. This was only implemented for the Tab class (not Inspector
or Task Manaager).
This patch adds LadybirdWebViewWindow to contain the bare minimum needed
to render a LadybirdWebView. All windows containing a WebView inherit
from this class, to ensure their WebContent processes know they became
visible.
This large commit also refactors LibWebView's process handling to use
a top-level Application class that uses a new WebView::Process class to
encapsulate the IPC-centric nature of each helper process.
