Reworked new code block editor using user control to fix port alignment and watermark issues

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 regression tests on the new CBN editor after these changes
- Existing bugs with misalignment of input ports when there are line spaces in the text

@Benglin please review