Add custom image during loading of the geppetto application

[ddelpiano]

• Custom image can be placed in the webapp/images/splash.png
• the custom image will replace the text "LOADING... [~3MB]" and displayed instead of it during the load.

This is just an extension of Jesus' branch but based on the latest development, so I opened a new branch from my local development.

[ddelpiano]

@Robbie1977 could you review the changes in the stackviewer please?
I had some conflict and resolved them but a second look will make me feel in peace, thx.

@gidili could you review the geppetto.ejs and if Robbie is fine with the stackviewer changes merge this please? Thx!

[ddelpiano]

address this

[filippomc]

@ddelpiano my only concerns in the code are about the Stackviewer stuff and the weird styling of the image.

Also I can't see the related issue in the PR nor in the geppetto-client project. Every pr should refer to the issue in the title and a command in the body (e.g. "Closes #500")