Improve Ignition Fuel support

[osrf-migration]

Original report (archived issue) by Ian Chen (Bitbucket: Ian Chen, GitHub: iche033).

---

a few issues identified in pull request #2800

• remove the need to set GAZEBO_MODEL_PATH

• make sure both server and client is able to download from fuel (currently only client downloads the model files via InsertModelWidget.cc)

• test saving world with models from fuel and opening the saved world

• test logging and verify playback on new machine

• support downloading models that contain references to other models

• support Fuel URIs specified in SDF: <uri>http://ignitionfuel.org/{owner}/models/{model}</uri>

[osrf-migration]

Original comment by Louise Poubel (Bitbucket: chapulina, GitHub: chapulina).

---

Also, support Fuel URIs specified in SDF, i.e.:

<uri>http://ignitionfuel.org/{owner}/models/{model}</uri>

[osrf-migration]

Original comment by Ian Chen (Bitbucket: Ian Chen, GitHub: iche033).

---

• Edited issue description

[osrf-migration]

Original comment by Ian Chen (Bitbucket: Ian Chen, GitHub: iche033).

---

• Edited issue description