Initial implementation of TextBlock.Inlines

[grokys]

This PR adds TextBlock.Inlines and a couple of TextElements:

• Run
• LineBreak

This allows one to format the text within a TextBlock, e.g.:

        <TextBlock>
          A <Run Classes="code">TextBlock</Run> with <Run FontWeight="Bold">differing</Run>
          <Run FontStyle="Italic">styles</Run> <Run FontSize="24">and fonts</Run>.
        </TextBlock>

Currently only works fully on Direct2D. The Skia backend currently only supports Foreground in FormattedText - it will need to be updated with the new properties introduced in this PR. If anyone would like to implement these in Skia, please do!

There are also more TextElements to add, such as Span and InlineUIContainer. These will come in a separate PR I think.

[Karnah]

If TextBlock contains only <LineBreak /> - this code throw exception. May be use "is IHasText"?

[grokys]

Nice find, as always! :)

[Karnah]

As I understand, you will not allow use TextElement into TextBlock?

[grokys]

Sorry @Karnah - I don't understand?

[Karnah]

I didn't know that in TextBlock I can use <Run /> and always format by another TextBlocks. Now I've tried and found out that you also can use TextBoxes in TextBlock in WPF!

<TextBlock>
    I see trees of <TextBlock Foreground="ForestGreen" Text="green" />, red roses too
    <LineBreak />
    I see them <TextBox Text="{Binding Bloom}" /> for me and you
    <LineBreak />
    And I think to myself what a <Run FontWeight="Bold" Text="wonderful world" />
</TextBlock>

It's hard for me to decide, is it necessary to support TextBlock and TextBox or not. But as I can understand, if you decide supporty this types, you should inherit them from Inline. Or am I wrong? Or this you'll implement in future? And I decided to clarify this :)

[grokys]

@Karnah ok, I think I understand: yes what you're asking for is InlineUIContainer which is an Inline which wraps controls. I mentioned this in the description above, and yes - we should probably have this if possible, though I'm not sure how difficult it would be!

[danwalmsley]

Should we just remove this anyway, since the next version is going to be 0.7 and we already have several other
breaking changes?

[Gillibald]

I have merged everything with the Skia text layout PR if someone wants to try this.
https://github.com/Gillibald/Avalonia/tree/feature/SkiaInlines

[grokys]

Closing as this is superseded by #2423