Fixed Hotkey for controlling background preview.

Adding a new property that take note of both boolean WatchEscapeIsDown and CanNavigateBackground for the hit test of the "Watch3DView".
Removed preview from nodes that are not suppose to have preview.
Merge pull request #730 from l2obin/HotkeyFixesBranch

Fixed Hotkey for controlling background preview.
Merge pull request #731 from SkyApple/RemovingPreviewFromSpecificNodes_ChocoButterUI

Removed preview from nodes that are not suppose to have preview.
Merge branch 'master' into ChocoButterUI
Changed code to fast exit.
Renamed "dynScriptEditWindow" class/files to "ScriptEditWindow"
1. Made "DynamoViewModel.ExecuteCommand" method public (accessible from DynamoPython.dll)
2. Made "RecordableCommand" and all its derived classes public (accessible from DynamoPython.dll)
1. Refactored common ScriptEditWindow specific codes out of Python nodes
2. ScriptEditWindow now directly owns/manages the Auto-complete window
3. ScriptEditWindow updates Python nodes through UpdateModelValueCommand
Implemented SerializeCore/DeserializeCore methods for Python/PythonVarIn classes
Fixed duplicated port registration in PythonVarIn.SerializeCore/DeserializeCore methods
1. Added three new test cases to completely test undo/redo for Python nodes
2. Exposed Python/PythonVarIn.Script read-only property for verification in test cases
Merge pull request #736 from SkyApple/AddingQuickExitToCode_ChocoButterUI

Adding quick exit to code choco butter ui
Merge pull request #738 from Benglin/PythonNodeRefactor

Fix: MAGN-566 Editing of python nodes cannot be undone/redone
Merge branch 'master' into ChocoButterUI

Conflicts:
	src/DynamoCoreUITests/RecordedTests.cs
Merge branch 'master' into ChocoButterUI

Conflicts:
	src/DynamoCoreUITests/RecordedTests.cs
Added "SnowPeashooter.xml" to introduce 0.5s delay so subsequent test cases pass
Merge pull request #744 from ambarish-prakash/ChocoButterUI

Accepting this to verify the build fix
Updated "PauseAfterPlaybackInMs" from 500ms to 2000ms (test timing fix)
Embedded OpenSans Font and create Common Font Variables in DynamoModern.xml