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#KDE has long had a technology called #KParts. Basically you can set up something (including a full app) as a #KPart component and use it elsewhere as part of another (tools bars and menus will merge).
My understanding is that #Kate & #KWrite are a good example. Both now use the same editor component, but Kate adds a LOT of extra functionality to it. The #KDEPIM suite uses the same trick with it's components - they can run separately or as one combined app (#Kontact).
I'm not fully across the technology, so if any experts wish to correct me, feel free. 