Allow the Select tool to partially interact with artboards

Right now, the Select tool doesn't let you interact with artboards (except for one undesired side-effect: it visualizes the yellow pivot gizmo at the artboard center when an artboard is selected). Only the Artboard tool lets you, but we want to replicate some of the Artboard tool's features in the Select tool in certain states.

If an artboard is (or multiple are) selected (from the Layers panel), we want the Select tool to display the transform cage bounds gizmos that we currently have with the Artboard tool. If both an artboard and a non-artboard is selected, we filter out the artboard(s) and only show the selection for the non-artboards.

When multiple artboards are selected, dragging the combined transform cage bounds should scale all the selected artboards proportionally, just as with regular rectangle layers. The only difference is that we have to round to integer values since artboards must be integer-sized. Currently, the Artboard tool only draws bounds around the most recently selected artboard even if multiple are selected, so that needs to change for both the Artboard and Select tools.

Selecting an artboard should be possible by double-clicking its background (in a location where no inner content is covering up the artboard background) or by single-clicking the artboard title which is drawn at its top-left. Holding Shift in both cases should allow adding/removing to the selection of artboards. Single-clicking an artboard background with the Select tool active should deselect it, just as it does currently in Graphite.