[GSoC] Visualization plugins placement

[Sarath18]

This ticket is concerned with how the visualization plugins are loaded and placed in ign-rviz.

Potential places to load visualization plugins:

1. Dialog window on the top-right of ignition gui.
2. Drawer Menu in ignition-gui. The save/load config options can then be moved to the dialog window.
3. Add, Remove, Duplicate button in a master visualization Displays GUI plugin (similar to RViz)

After loading the plugins, where are these visualization plugins placed in the application GUI:

1. Each plugin is loaded individually with their GUI configs and is split horizontally adding new visualization plugins (like GUI plugins in ign-gui)
2. All the visualization plugins after loading are placed in a master Displays GUI plugin and can be configured from there. (like RViz)

In the scenario where multiple visualization plugins are loaded, option 1 might tend to clutter the GUI. I am not sure if option 2 is feasible.

@ahcorde, @chapulina can you help me figure out the right implementation for this problem?