Improve GUI in 3D demo project

[rpaciorek]

• fixed GUI input issue in billboard mode (by use custom billboard calculation also for mesh node)
• added settings for gui_3d script gui_panel_3d.tscn allows configure gui in 3d without modifying this scene (and easily reuse it)

I'm not sure if this approach (with configuration via setters in gui_3d.gd) isn't too complicated for a demo ... but:

1. managing the billboard at the script level (and not the material level) allows solving the input synchronization problem by applying rotation to the entire node (and not just collisions) ... this shouldn't cause a significant overhead because these calculations were performed and applied by this script anyway ...
2. separates the configuration from the logic of the 3D GUI object, allowing you to easily reuse this solution in your project