Enable Text widget

[nicoburns]

There was already a text widget in the repository. This PR:

• Enables the text view and widget modules
• Fixes them up so they compile with latest Xilem
• Implements compute_max_intrinsic for text

I'm not sure that having String for the view makes sense because:

• Strings are not string literals, so the DX isn't that great anyway as you have to wrap in String::from, which is longer than some hypothetical text function for a custom text type.
• We're almost certainly going to want modifiers on text like font size, color, etc. And we'll need to store those somewhere.

But I've left it as String for now.

[Philipp-M]

Strings are not string literals, so the DX isn't that great anyway as you have to wrap in String::from, which is longer than some hypothetical text function for a custom text type.

We'll likely need all kinds View impls for string based views (&str, Cow<str> etc.), and I think they may share all the same widget, and maybe even wrap the same text view.

We're almost certainly going to want modifiers on text like font size, color, etc. And we'll need to store those somewhere.

I think a few composing style views probably make sense for such views (and maybe in general). Something like StyledText<String> or StyledText<&'static str>.

I'll try to review this later (or in the following few days).

[Philipp-M]

Apart from the one comment, this looks good to me, thanks, finally support for strings :)