static struct trwlayer_interface = {
 ...,
 trwlayer_tools,
 sizeof ( trwlayer_tools ) / sizeof ( LayerTool ), /* gshort */
};

static struct trwlayer_tools = {
  { "Add Waypoint", trwlayer_add_wp },
  { "Add Track", trwlayer_add_track },
};

----

Tools Adding:
ItemFactory args:
1) VikWindow
2) 4-byte variable containing ID and ID of tool.

struct VikLayerToolId {
  gshort layer_id;
  gshort tool_id;
};

Then we just need to assert (or #warning, etc.) that 
sizeof(VikLayerToolId) <= sizeof(gpointer)
OR, I could use guint8 to be on the safe size, for a maximum of 256 
layer types. But I think guint16 will be file.