Add two new tools, GimpGuideTool and GimpSamplePointTool. They are
one-trick-ponies and can only create new or move existing guides and
sample points. They can't be selected from the toolbox, only
temporarily pushed as active tools on top of any active tool using
their public start() APIs.
Use that API to enable them when the rulers are clicked, and replace
the entire guide and sample point moving code in GimpMoveTool and
GimpColorTool by simple calls to that API.
This might look like overkill but can easily be used for other
features like moving guides from within the paint tools (mirror
painting) or gegl filters (preview curtain).
Michael Natterer