Enhancement: Implement Layer Editor add remove layers

[BigRoy]

Implements:

• You can now right click a sublayer to remove it from the layer stack - fix Layer Editor: Implement the add and removal of layers #2
• You can now right click "add layer" to pick files from disk to add as sublayers - fix Layer Editor: Implement the add and removal of layers #2
• You can now drag an drop usd files from your OS file explorer (e.g. Windows Explorer) onto the layer list

Fixes:

• This also fixes an issue where the same layer identifier across the layer stack (e.g. present in different sublayers) would error in the model on the uniqueness of the key - those are now made unique by using the key {parent.identifier}-sublayer->{layer.identifier}
• Some minor cosmetics - some print statements are now log messages.