Added a sidebar to the main document window in macosapp that lists the layers in the current style and updates whenever the style changes or a layer is added or removed programmatically. Display an icon beside each layer in the sidebar that indicates the kind of layer. Double-click a layer or layers to toggle their visibility, which is an undoable action. Added a menu item and toolbar button to toggle the Layers sidebar. Added a context menu for toggling visibility of and deleting layers selected in the Layers sidebar.
Checked in the original SVGs for layer icons.
