[Proposal] Add layerDelayedLock capability

[xrigau]

This adds a capability to switch between layers that acts like a 'lock'
but with the difference that it requires the 'lock' key to be pressed
for 500 milliseconds before the function layer gets locked.

This is useful to avoid switching to a function layer by mistake since
pressing the lock key by accident won't actually switch layer unless
it's held for at least 500ms. (and yes an improvement would be to make those 500 milliseconds customizable).

So essentially:

• Keyboard is in default layer 0.
• User presses layerDelayedLock(1) key
  • If the user releases the 'delayed lock' key before 500 milliseconds, nothing happens, the current layer will still be layer 0.
  • If a new key is pressed before 500 milliseconds, the key press will still be from layer 0.
• After 500 milliseconds have elapsed since the 'delayed lock' key was pressed then the keyboard locks to layer 1
  • The same behaviour applies to unlock the layer.

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This is a proposal, perhaps it's not the best way to implement it but I've been using it on a daily basis for about a month now and it works well for me. Feel free to close the PR if you don't think this feature fits well in the project.

Also note that I don't usually code in C/C++ so this can probably be improved by someone with more knowledge.