Change cb ntext editor

This submission fixes the following issues in porting the existing code block node to the new Avalon based Editor:

Port height and line height in text editor mismatch – AvalonEdit does not provide a way to control line heights
Watermark was not working
Borders around new editor UI were not visible
The above issues are resolved using a new approach - custom (UserControl) control container around AvalonEdit UI. See files:
1. CodeBlockEditor.xaml
2. CodeBlockEditor.xaml.cs

Note: This PR includes the implementation for basic syntax highlighting as well.

Pending tasks:
- Run UI automation tests on the new CBN editor after these changes and fix regressions if any
- Fix existing bugs with misalignment of input ports when there are line spaces in the text

@Benglin  please review