Add SDF elements for transparency, wireframe, etc...

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/217)) by Peter Mitrano (Bitbucket: [peter_mitrano](https://bitbucket.org/%7Bf79f793e-4f31-4c61-9df2-190b239e0bf2%7D/)).**

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# Summary

I would like in my world file to indicate that gazebo should start as if I went to `view`/`transparent` in the menu.

# Motivation

I am using the gazebo marker to show some markers, and sometimes those markers appear inside obstacles. This means I often start gazebo and then immediately set the view to transparent, so I can debug things. I would expect to write something like:

‌

```xml

0 0 25 0 1.5707 1.5707


true
true
... anything else in that menu



model://ground_plane


model://sun

...

...

```

# Describe alternatives you've considered

Make the actual models transparent, but then I would need different models for debugging or not which is bad.

Start with the SDF world-file example in the issue and trace how the and proposed elements are represented and parsed. Define the supported transparency, wireframe, and related view settings, then verify that a world file can express them unambiguously.

Written by the indexing model from the issue text.