Support Task driven Window movement.

[tmontes]

Notes:

• Window movements leave whatever is/was under the window now visible: maybe just a bunch of nothing.
• This will require some re-rendering.

Ideas:

• Simple approach: re-render everything.
  • Needs either the window-render trap to be told that or the kernel to somehow remember rendered window position, detecting such changes.
• An optimised approach will only re-render the changed bits.
  • Probably needs to be line oriented.
  • Only cleaning up a sub-set of the the kernel Terminal lines and fully rendering Windows overlapping them.

[tmontes]

Window movement is implemented and working well enough.
Documented a limitation in #91.