Add support for user-triggered kernel monitor tasks

[tmontes]

Idea:

• User hits a given key.
• Kernel toggles a task:
  • Spawns it if the task associated to that key isn't running.
  • Stops it otherwise.

Driving idea:

• Hitting a key and getting a Python REPL at any time.

Extra craziness:

• Maybe such tasks could consult some "global shared state" to determine their behaviour:
  • Where to position their windows (left? right? top?).
  • Which process to launch (Python REPL on a Python slide, Ruby REPL on a Ruby slide, etc..)

[tmontes]

Simple user-space only solution:

• Top task tracks the keyboard, directly or via a sub-task result / message.
• Spawns / destroys tasks "as needed".

No need for additional kernel support.

Given this, I'm moving this issue to TBD (maybe we'll never need this).