[Vala] Support for more attributes in vala properties.

[Diego Dompe <diego dompe ridgerun com>]

Hi all,

We have been working on implementing gstreamer plugins with vala, and we found we need some extra attributes 
for vala properties.
On GStreamer elements the min, max, and description on the param specs of an element proprerties are show by 
gst-inspect and provide useful information about the control properties of the element. However I can't set 
the mix, max or description of the properties on vala.

What would it take to get this added? We can work in a patch to support them, but I suspect vala parser gurus 
will be more efficient at it. Also I guess there need to be some agreement on what the syntax for the 
attributes would be?

Regards,

Diego