Preview Panel 2.0

[Zehir]

From #526

Whats next ?

This could be improved in a new PR in various ways;

• Update the result to display millisecond instead of second when the second is = 0
• The property preview_chunk_count is not currently used. (I am waiting for Improve GaeaTaskPool scheduling #545)
• Display the preview seed into the top bar of the GraphEdit with a refresh icon to generate a new one
• Remove the generate preview button to auto generate when changes happened in the graph (maybe with some delay because of performance issue)
• Find a way to avoid the blicking when re generating the chunks.
• Add a vertical range editor on the right side to show/hide layers in the same way 3D printer software does.
• Do something about the brown ground, the colors get reflected to the bottom side.
• Update the preview settings for each demo
• Add a way to update both generation and preview seed so we can do a complete generation in one click.