Contributed Guest Additions, with possibility of Net Install

[ajmas]

Discussion of how to provide WineD3D for VirtualBox (see ticket ​2940), suggested that there may be a need for the concept of 'contribued guest additions', which aren't necessarily tied to the build cycle of VirtualBox.

The concept is similar in many ways to Firefox and the extensions available for it. Basically you would have a library of optional guest additions, that can be downloaded and installed into VirtualBox. So as not to tie them to the build cycle of VirtualBox, the guest addition installer would check online to see whether there are any new optional additions and whether there are any updates. There could even be a start-up task that would periodically check to see if anything has changed, similar to Firefox.