Merge pull request #666 from ambarish-prakash/CodeBlockNodeWatermark

Code block node watermark
WaterMark and Escape(without undo fix):
Watermark : Addition of new style in DynamoModern xaml file. Integration of style with codeblock node
Escape : Handlers for onkeypress, onlostfocus, ongetfocus etc that handle deletion of node as well

Only partial deletion - deletion not completely fixed
Have also set textbox height to auto (instead of 20 set by master)
minor changes for focus