Compositor Layout Update with Image Editor>Viewer Node

Updating Compositor Default Layout with Image Editor>Viewer Node

Reason:

1. Compositor is node based means it's a jungle. Backdrop adds more to that jungle.
2. More professional look and feel with industry proven layout.
3. Image editor has Scopes, Color picker, color inspector. Crucial for compositing color grading.
4. It is unknown to new users that Image Editor>Viewer Node can be used instead of backdrop.