Add optional active control in libnotify bubbles

Currently Ubuntu (11.10) notification bubbles can contain graphics and text (summary + body). In this context the bubbles grey out when the cursor hovers it, to allow the user to interact with the desktop behind.

I think it would be really helpful to add a second scheme, in which GUI controls can be added (hyper-links, buttons, etc.), to allow the user to interact with application events. In this scheme the bubble should not grey out on mouse over.

In this way the integration of applications that use custom notification frames could be improved (e.g. Skype notification to accept/reject calls or Firefox prompt for completed downloads).