Theming node editor suggestions.

Here are few suggestions for Node Editor Theming

-Set drop-shadow off or alpha control to it

• Change shadow color

Similar to menus proposal: developer.blender.org/D754

• Color theme.node_editor.node_outline in order to change non selected or active nodes outline color

Following theme.node_editor.node_active and theme.node_editor.node_selected name convention should be theme.node_editor.node but I think is a little bit vague, so maybe name and rename to theme.node_editor.node_outline theme.node_editor.node_selected_outline theme.node_editor.node_active_outline

• Node outline size to control width of outline

• Backdrop could be changed when a node is selected or active, in order to have better wide view of what you are selecting.