Code provided below can be compiled using Arduino IDE provided by DigiStump. The firmware contains a neat trick allowing to cancel a press on the button. In fact you'll have a 2 second countdown before shortcut is sent. If emergency button is released within this period, action is canceled.

#include "DigiKeyboard.h"
#define KEY_DELETE 76

bool state = false;
int countdown = -1;

void setup() {
  pinMode(2, INPUT);
  digitalWrite(2, HIGH); // Pull-up
}

void loop() {
  // Kept from sample
  // this is generally not necessary but with some older systems it seems to
  // prevent missing the first character after a delay:
  DigiKeyboard.sendKeyStroke(0);

  int sensorValue = digitalRead(2);
  if (sensorValue == 1) {
    // Push button is pressed
    if (countdown == 0) {
      // It's the final count down ! Send shortcut Ctrl-Alt-L
      if (state == false) {
        DigiKeyboard.sendKeyStroke(KEY_L, MOD_ALT_LEFT | MOD_CONTROL_LEFT);
        state = true;
      }
    } else if (countdown == -1) {
      // We were waiting, start countdown
      countdown = 20; // countdown * 100 = <time in milliseconds>
      state = false;
    } else {
      // Still pressed, continue the count down
      countdown--;
    }
  } else {
    // Push button is released (Short circuit -> GND)
    // Reset
    countdown = -1;
  }

  // Kept from sample
  // It's better to use DigiKeyboard.delay() over the regular Arduino delay()
  // if doing keyboard stuff because it keeps talking to the computer to make
  // sure the computer knows the keyboard is alive and connected
  DigiKeyboard.delay(100);
}