Move window edges in `SliceLayoutEngine`

[dalyIsaac]

• [ ] Resize areas inside ParentArea
• [ ] Resize windows inside an BaseSliceArea?
• [ ] Move window edges in direction (just parent vs. all windows)
  • [ ] [increment|decrement]_master_width command: resize width of master column. If there are columns to either side of the master column, then this would equally shift the edges on both sides.
  • [ ] [increment|decrement]_current_column_width command: same as the *_master_width commands, but it operates on the currently active column.