Markdown.xaml enhanced

[MaxAquila]

Hi I'm Massimiliano,
I have made some changes to the main projects that I'd like to share, so I open this pull request.

• Clean code and styles.
• New separators.
• Six heading style and edited old font-size.
• New list markers and increased the nested limit to 6.
• New table style with an alternative background color for the rows.
• Edited code span and added code block.
• Added tooltip to images and default image support when no image can be loaded.
• Support for colored text, with brushes or hex.
• New text decorations: underline and strikethrough.
• Support for text alignment, for document and/or single paragraph.

I also created an editor in the Demo project that works like the previous one (Live mode) and in manual mode; pressing the "Preview" button you will see the final result.
In a secondary tabitem there is a read-only DEMO, where you can see almost all the features of markdown.xaml.

[theunrepentantgeek]

Thanks for the PR! I'll review this as soon as I get an opportunity.

[MaxAquila]

New commits:

• Added roman numerals marker for lists (lower and upper).
• Added block note.
• Added blockquotes up to 9 nested.