Integrate with GTK Broadway

[nintyfan]

Allow to run applications in widgets on desktop. On one widget could been displayed many applications. Alternatively, user could undock this widget to obtain separate window.

Why? Because I will add semantic information to both Qt and GTK, so web backends could provide it to browser (in special attributes), so you could parse window with XSLT, CSS. For example you could move toolbar to top of desktop. This is only one use case.

[codesardine]

Running inside widgets makes no sense as will require a browser window when we can just open the application itself in their native window.

QT does not support broadway backend https://developer.gnome.org/gtk3/stable/gtk-broadway.html.