This commit adds warning messages to "legacy" nodes that will be removed in the future. The warning is shown in the node header, but it is not printed in the terminal or displayed in the modifier. It is also not propogated to node groups, but that is a more general task. If the modifier's node tree has executed a deprecated node, it will display a warning and a "Search" button that will select the nodes and pan to them in the node editor. This doesn't open child node trees and select nodes in there, because I want to keep this operator simple and avoid wasting a lot of time perfecting this behavior. Differential Revision: https://developer.blender.org/D12454