Support loading more than one visual plugin on GUI side

[iche033]

Environment

• OS Version: Ubuntu 20.04
• Source or binary build?
  binary and source, ign-gazebo6

Description

Visual plugin support on the GUI side is added in #1275. However when multiple visual plugins are specified for a <visual>, only the last one will be loaded, see
#1335 (comment)

• Expected behavior: ign-gazebo's gui loads multiple visual plugin per visual
• Actual behavior: ign-gazebo's gui only loads the last visual plugin

This is mainly because we are only creating attaching one VisualPlugin component to a visual entity on the server side, which then get serialized and sent to the GUI side:
https://github.com/ignitionrobotics/ign-gazebo/blob/0dead0578ba3f26cf97f8162e55a0dbe1112484c/src/SdfEntityCreator.cc#L752-L761

A potential fix is to use component that stores a list of visual plugins instead

[chapulina]

A potential fix is to use component that stores a list of visual plugins instead

We'll need some way of passing system information from the server to the client for #191. I was thinking of creating a component that stores a ignition::msgs::Plugin_V. Do you think that could address all the needs of the VisualPlugin, @iche033 ? I think that would be better to serialize than an sdf::ElementPtr.

[iche033]

I think so. As long as contents of the plugin.proto msg can be converted back to an sdf::ElementPtr then it should be fine. For loading visual plugins, it comes down to this call on the GUI side:
https://github.com/ignitionrobotics/ign-gazebo/blob/ign-gazebo6/src/gui/GuiRunner.cc#L343-L344