add Speed2 ! to cancel pending one-shot speed setting

[dgwynne]

Description:

Speed2 ! can be used to cancel the use of a preceding Speed2 command. This is useful if you're using Setoption20 (update of Dimmer/Color/CT without turning power on) so you can control things like the colour temperature of light from a cron job or schedule without the light turning on when you change the CT. However, if you have Fade 1 and Speed 2 as your normal speed/fade settings, and you change the CT with a command like Backlog Speed2 20; CT 500 to get a gradual fade when the light is on, the Speed2 isn't "consumed" until the light is turned on. When you turn the light on later, it takes 10 seconds instead of 1.

With Speed2 !, you can use Backlog Speed2 20; CT 500; Speed2 ! as the scheduled command. If the light is on, the Speed2 10 is used by CT command immediately because the light is already on. If the light is off, the CT command doesn't take effect, leaving the preceding Speed2 change ready to go when the light is actually turned on. The Speed2 ! clears that, so when you do turn the light on in the future it acts as expected (with the Speed 2 setting).

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works with Tasmota core ESP32 V.2.0.4
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass