add option to specify which layers the layerbrush will select

[alipha]

This enhances the Layer Brush so that you have the option to specify which layers you affect when you select and move a region. By default, all layers will be selected and moved. However, for instance, maybe you have multiple layers like:

• Ground Layer
• Ground Decorations Layer
• Solid Layer
• Solid Decorations Layer
• Foreground Layer

If I'm moving a house in a town (in a top-down game), then I probably don't want to move the Ground Layer and Ground Decorations Layer. And so, this PR, for instance, allows me to specify I only want to move the Solid Layer, Solid Decorations Layer, and Foreground Layer.

cc: @marcus-robinson

[unity-cla-assistant]

All committers have signed the CLA.