✨ Split Sidebar Panels

## Is your feature request related to a problem? Please describe.

Users might want to see more than one sidebar tab at once.

## Describe the solution you'd like

Drag a sidebar symbol and drop it over the lower section to split the sidebar panel. User is then allowed to drag other symbols into that panels dock.

## Mockups

### Step 1: User mouses over to sidebar item that they would like to go in a new split panel

### Step 2: User drags symbol to lower half of the sidebar

Notice that the placeholder radius follows the window radius.

### Step 3: User releases mouse and split panel is added with symbol in lower panel dock

### Step 4: User drags additional item to lower sidebar panel

## Additional context

When user is dragging symbol and sidebar panel is not split yet, when crossing over the halfway threshold, haptic feedback is felt if the user is using a trackpad.

Split sidebar panels will be resizable.

A sidebar panel can only be split once for a maximum total of two sidebar panels.

This issue is concerning both the navigator sidebar (left) and the inspector sidebar (right).

Refer to #41 for more information.

## Related Issues
- #338