Markdown reloads when component classes change.

[rodrigogiraoserrao]

Fixes #3464.

The reason why these component classes weren't being reloaded was that markdown blocks inherit from Static and they had their content set when markdown was first parsed, and then they would lose access to the source markdown.

This PR moves some of the logic that builds markdown blocks into a MarkdownBlock method that parses the markdown tokens into a rich.text.Text but also keeps track of the original source token.
This way, when CSS is reloaded, we can just ask the MarkdownBlock to rebuild itself.

By creating this method directly in MarkdownBlock, we only save the markup that is necessary and only reload the nodes that may have inline styles.

[rodrigogiraoserrao]

From style_stack: ... to the end of this method, I copied and pasted the code.
The only change happens when I'm getting the component classes.
Instead of getting them from self, I get them from self._markdown.