Speed up resource spawner load time

[chapulina]

Environment

• OS Version: all
• Source or binary build? all

Description

• Expected behavior: Resource spawner GUI plugin shows models soon after being loaded
• Actual behavior: It shows "Please wait... Loading models from Fuel" for minutes sometimes.

One idea for a fix would be to load the Open Robotics models first, and then all models from other users subsequently. We should populate the menu while more models are loaded on the background, instead of waiting until all models from all owners are loaded. We may need to add APIs to ign-fuel-tools for that.

Steps to reproduce

1. ign gazebo
2. Insert Resource Spawner from top-right corner
3. Wait