Use LLSD/XML for exported body part format

In 1.2, body parts are exported as a really stupid file format, like this:

type 0
parameters 82
1 .23
2 -.37
4 -.23
5 .03
6 .57
7 -.09
8 -.08
10 .12
11 -.12
12 .19
13 0
14 .1
15 0
17 .01
18 .26
19 -.28
20 -.16
...

We should change it to an LLSD-based format (or other XML) instead.

For backwards compatibility, the viewer should still be able to import the stupid old format as well as the new format.