Currently, if we want to add a new e.g. WebContent command line option,
we have to add it to all of Qt, AppKit, and headless-browser. (Or worse,
we only add it to one of these, and we have feature disparity).
To prevent this, this moves command line flags to WebView::Application.
The flags are assigned to ChromeOptions and WebContentOptions structs.
Each chrome can still add its platform-specific options; for example,
the Qt chrome has a flag to enable Qt networking.
There should be no behavior change here, other than that AppKit will now
support command line flags that were previously only supported by Qt.
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.
