After cleaning up the native flags in various places, and disabling making sibling of native widgets native, BrowserDock was left in an inconsistent state, since it requires being native for CEF to work properly. Make BrowserDock native.
Shuts down the actual browsers internally when the browser docks are hidden by user.
Refactors all cases where a window class was created to show a browser dock in to one class: BrowserDock. Removes some unnecessary code duplication.