[Feature] Support for subgroups in a Scene

[apoapostolov]

Inside a single scene or playlist, allow to create groups (like panels containing multiple sounds) with title an color tint customization. Each Group can be thematically used by DM to create "combat music", "background music", "day" and "night", etc. without having to switch to another scene for this. Especially on full screen use, there's usually a lot of space to do this in a three - four - dynamic number of columns to display these groups, like a kamban board.