Implement Toggler widget for iced_native

[Kaiden42]

This PR implements the Toggler widget mentioned in #528 . The Toggler widget works similar to the Checkbox widget.

I am open to suggestions for improvement. Currently iced_web is not supported, but maybe I can check it out in the near future.

Closes #528.

[clarkmoody]

This seems like a great start!

What I would like to see:

• More flexibility when it comes to the label. Right now, we've got a String on the left side of the toggler. I'd like to see a generic impl Into<Element<'a, Message>> as the label, with the option of placing it on the right or left side.
• Maybe instead of the previous, we simply add a with_toggler helper method that joins the elements to a toggler, and the toggler itself becomes a standalone widget without the label.
• Either an example program showing off the toggler, or an update to the tour example with toggler in there

[Kaiden42]

Thank you very much for reviewing my PR.

• I agree, the label needs more flexibility. However, I don't think taking an impl Into<Element<'a, Message>> instead of a String would be the right choice here. I can't imagine a valid usecase for e.g. a Slider to be the label for a Toggler. This doesn't sound to me. I could add a function for specifying the alignment of the label instead.

• As mentioned in the previous point, only a Text would be a useful element for a label. Maybe making the label optional for a stand alone Toggler would be more useful instead.

• Alright! I could update the tour and / or maybe the styling example(s), showing how to use (and style) the Toggler widget.

[clarkmoody]

You might want a small SVG image to be the label instead of a String of text.

If you want to bold a portion of the label, you'll need more than one Text joined with a container.

I think it's more useful, like you say, to avoid the label altogether, or use Option<String>.

[hecrj]

Let's keep things simple!

I think it's fine to satisfy only a subset of potential use cases. I prefer to tackle a small amount of specific use cases directly instead of a flexible API that can be used wrong.

That said, I believe removing the label would simplify the widget considerably. We should probably come up with a Label widget of some sort eventually.

[Kaiden42]

I've now made the label optional and added a function to specify the alignment of the label. Specifying the alignment of the label can be usefull when using a 'right to left' language instead of a 'left to right' language.

In addition, I've also added the Toggler to the styling and tour example.

Currently the default width of the widget is Length::Fill. This is needed for a good looking alignment with a lable especially when using multiple togglers in a Column. However, when avoiding the label Length::Shrink would be a better default value. Should I add some logic for choosing the default value of the width to Length::Fill when using a label and Length::Shink when not or would this just be to confusing?

[Kaiden42]

I've now added the Toggler widget to iced_web as well. However, I'm unhappy about the css styling. Maybe someone with more experience in web developement could take a look into it.

[hecrj]

I have rebased on top of master and made a couple of small changes.

I think we can finally merge this! Thank you @Kaiden42! 🎉