WIP: Adds basic Linux support, it's building...

[erik-nilcoast]

I'm not entirely sure how to test this yet, but it does compile now and open in the browser. I need to find the equivalent to setView for the X11 environment, which may be more involved than the simple API provided by macOS and Windows.

[nick-thompson]

Fantastic @erik-nilcoast thank you so much! I took a quick pass and generally this looks great. I see what you mean on the view handle piece. I saw this bit in the JUCE docs for that XEmbedComponent:

This is how you embed a GTK+ widget: if you are using the client initiated version of the protocol, then create a new gtk widget with gtk_plug_new (0). Then query the window id of the plug via gtk_plug_get_id(). Pass this id to the constructor of this class.

So I think perhaps, call getViewHandle on the webview, then try to cast it to GtkWidget* (as shown here: https://github.com/Tracktion/choc/blob/main/gui/choc_WebView.h#L332), then perhaps we can use gtk_plug_get_id() from that GtkWidget*? And if so, we can pass it into the constructor for the XEmbedComponent?