Added select_all method to TextInput.

[AldoMX]

Hi, and thank you for creating iced.

I was tinkering with the todos example and I wanted to pre-select the text when you clicked the edit button.

But when I tried to update the example I noticed that the public methods from TextInput only allowed to move the cursor, so I added a select_all method and updated the todos example to achieve the desired outcome.

[hecrj]

Works great! Thank you 🎉

I have added the method to iced_web too, although we can't properly implement it yet! This way, the todos example can still be built for web.

[derezzedex]

I've left a comment regarding the proposed function. There's also the need to implement this on the web backend, but I'm not sure how that works because I'm not familiar enough with it.

[derezzedex]

I think it would make more sense to expose a more general select function, accepting both the start and the end, then in case you needed to select all you would use it just like you currently call cursor.select_range.

I'm not sure if there's a way to clip the range to the actual text size, since the Value is not available for the State, but right now this is done anyway on the Renderer: https://github.com/hecrj/iced/blob/f076649fbb575ee036ab0b3f4511690e3379c115/graphics/src/widget/text_input.rs#L157-L158

[hecrj]

I think select_all satisfies a clear use case.

We can expose a select_range or similar and focus on those use cases in a different unit of work.