Collada import

[cochrane]

Closely related to #67: It would be very nice if one could also load Collada files. But this raises a lot of issues, since Collada files come in a lot of varieties, so parsing them all can be a challenge - especially when it comes to exporting again. The Collada specification has some very strict requirements about any tool processing Collada files, which I could probably not easily meet. So this is not a priority at the moment.