Check for incoherences in available action button for actions

[nrueff]

Amongst the wide range of possible command states:

• "failed" and "done" are considered as being unmovable, neither start or stop may be proposed for such a command,
• "Play" must only be proposed for "scheduled", "stopped" or "paused" commands: clicking on it asks the scheduler to poll the command, altering state (=> "scheduled") and next_launch_date (=> "now") if needed.
• "Stop" may only be proposed for commands not "stopped", "done", "failed", asking the scheduler to stop the command and altering state (=> "stopped").
• "Pause" has the same effet as "Stop", without killing the command.

[oroussy]

|---------------|------|------|-------|
|               | Play | Stop | Pause |
|---------------|------|------|-------|
| failed, done  |  X   |  X   |   X   |
| stopped       |  O   |  X   |   X   |
| paused        |  O   |  O   |   X   |
| *_in_progress |  X   |  O   |   O   |
| other         |  O   |  O   |   O   |
|---------------|------|------|-------|

[Rock]

Haha, shouldn't you be charingg for that kind of knowledge?!