[FEAT] Select multiple elements at once

[drfarrell]

Allow a user to hold down SHIFT and select multiple elements on the canvas and / or layers at the same time. The multi-modality is important because some elements may be out of view but in the layers list.

Requirements

• [ ] When a user selects one element in the layers panel, it also selects that element on the canvas, and vice-versa
• [ ] When a user stops holding SHIFT, they can still apply properties to multiple elements, but if they click on a single item or away from the multi-selected group, the elements unselect.

[Kitenite]

@drfarrell I mentioned this but we should have some cases defined in the requirement such as how to display properties on the editor panel when multiple elements are selected. When happens when elements of different types are selected, etc.