Add support for megawatt

[Maschga]

Hi,
this PR adds support for the unit megawatt and its display on the screen.
If support for gigawatt is wanted I can add this, too.

I also changed the tests to work with the new enum.

I was unsure how I should export the enum.
If the current solution is not correct, please let me know so that I can learn from it and improve it.

Also, I was unsure if I should rename this function:

powerInKw(newVal, oldVal) {
	// force update if unit changes but not the value
	if (newVal !== oldVal) {
		this.$refs.powerNumber.forceUpdate();
	}
}

Fixes #15290

Thank you very much for this great project!
~ Maschga

[naltatis]

👍 Looks really good. You've picked a first issue that touches a lot of different parts of the UI :D
I did a quick scan through your changes and left comments. I'll give it a more in dept look and try and test different scenarios at a later time.

[naltatis]

Looks good to me. We can merge once the last comments are done (and the tests are green).

[naltatis]

Thanks! 🙌