Reorganize folder structure for the visualizer's plugins

[francocipollone]

After ignition migration is completed #332
the visualizer folder should be reorganized in order to be more clear which files are from which plugin.

e.g.: Each plugin's files should be located within a particular folder named as the name of the plugin.

[francocipollone]

The ign-gui0 based plugins aren't targeted to be reorganized because they are going to be deleted shortly