Hold Cmd-Q to Quit as a system-wide action?
You can do this by installing and running Hammerspoon, then including the following Lua code in the Hammerspoon configuration file (.hammerspoon/init.lua):

-- config: number of seconds to hold Command-Q to quit application
cmdQDelay = 2

cmdQTimer = nil
cmdQAlert = nil

function cmdQCleanup()
  hs.alert.closeSpecific(cmdQAlert)
  cmdQTimer = nil
  cmdQAlert = nil
end

function stopCmdQ()
  if cmdQTimer then
    cmdQTimer:stop()
    cmdQCleanup()
    hs.alert("quit canceled",0.5)
  end
end

function startCmdQ()
  local app = hs.application.frontmostApplication()
  cmdQTimer = hs.timer.doAfter(cmdQDelay, function() app:kill(); cmdQCleanup() end)
  cmdQAlert = hs.alert("hold to quit " .. app:name(), true)
end

cmdQ = hs.hotkey.bind({"cmd"},"q",startCmdQ,stopCmdQ)


Change the value of cmdQDelay to fit your preference.

A brief explanation:

The final line tells Hammerspoon to intercept any presses of Command-Q. It will run the lua function named startCmdQ when the key is pressed, and stopCmdQ when it is released.

startCmdQ takes a note of the foreground app, then starts a timer. If the timer times out, the function given as argument to the timer is called. It kills the app (really, asks it to quit) and cleans up after the action.

stopCmdQ stops the timer if it is still running, so the application is not quit after all. It too cleans up afterwards.

The action is accompanied by alerts to let you know what is going on.

Edit: Once this is installed and active, you can turn it off by running cmdQ:disable() in the hammerspoon console. Turn it back on with cmdQ:enable(). And if you want to excempt some apps from the delay treatment, you can add a test for those apps in the startCmdQ function, opting to run app:kill() immediately instead of starting the timer.

Edit the second: A word of caution. This will let you quit the Finder. Don't panic, though. The easiest way to relaunch Finder is to click on its icon in the Dock. Or you can use Spotlight: Open it via the magnifying glass icon in the menu bar or its keyboard shortcut and type “Finder.app”, then return. (You will only need to type a few letters, as Spotlight will fill in the rest for you). You can also relaunch Finder by running the command hs.application.open("Finder") in the hammerspoon console, or open -a Finder in a Terminal window, if you happen to have Terminal running.