Better interaction for background/layers

[rmpowell77]

Background images should be more like dots, I shoudl be able to select it same as I would with a dot.
Then there should be a way of making the background image "locked", meaning it is unselectable. Should also have a way of making it invisible.
Should be able to add more than one background image, and move them around individually, and selectable.

[rmpowell77]

Maybe take a page from Adobe illustrator, where there's a sidebar/floating window that controls visibility.

[noahsark769]

We should add the concept of layers and things (including paths and background images and dots) being able to be put on different layers. Also layers should be lockable.

[rmpowell77]

I would say this was addressed in #175.