[Window] Fix leak of whole dialog/window content.
[Window] Fix leak of whole dialog/window content.

Basically, this disposes of the Window content in the Xwt hierarchy.

Previously, this was insufficient, while it _did_ remove the Gtk
references, subscribing to an eventhandler and overriding Dispose on a
widget that's placed in a Xwt Window will cause the widget to not be
disposed, thus leak memory.

lytico/xwt,hamekoz/xwt,residuum/xwt,hwthomas/xwt,mono/xwt,antmicro/xwt,cra0zy/xwt,akrisiun/xwt,TheBrainTech/xwt