Stack layout tracker

[dalyIsaac]

Tracker for #885.

The following changes should occur with stack functionality turned off, unless a build flag is set to enable it:

• [ ] #1077
• [ ] Adding/removing windows from a stack, via commands:
  • [ ] Move window to stack in direction
  • [ ] Unstack window
• [ ] Follow-up commands:
  • [ ] Focus next window in stack
• [ ] Focus previous window in stack
• [ ] Focus adjacent stack in direction
• [ ] Drag-and-drop to stack
  • [ ] Holding down a modifier key to stack
  • [ ] Holding down a modifier key to move not stacked
  • [ ] Dragging a window to the center of the stack

Once the core functionality is implemented, the stack functionality can be enabled by default, and the plugins can be updated to use it:

• [ ] LayoutPreview
• [ ] Bar