Add slm_remove and slm_update listeners

[eighty20results]

Patch against v4.0 to add slm_update and slm_remove handlers.

Both listeners require the license_key to complete.

slm_update will let you update/change the expiration date, for instance. This is useful when dealing with automatically renewing payments (can set the expiration to the payment date and then update when the renewal payment is confirmed).

slm_remove is nice to have when needing to clean up user data (GDPR), etc.

Add_SLM_UPDATE_and_SLM_REMOVE_for_Software_License_Manager.patch.txt

[Arsenal21]

Hi, Thank you. Please create a pull request. That will let us compare the code changes easily and merge the code.

[eighty20results]

In #17