Proper power-button behaviour

Right now, pressing the power button immediately triggers a reset. This is nice for development, but not really the intended behaviour for the final release.

@schneider: Can you summarize what you had in mind?

added 3-Epicardium label

added 1-Necessary label

For now, my idea:

• Short press (immediately): Stop the running payload and return to the menu (#26 (closed)).
• Keeping the button pressed for 1s: Go into sleep
• more than 5s: Actually power off

mentioned in commit 97701165

closed via merge request !16 (merged)