I18n: disambiguate a few translations

- Keying (keyframe insertion)
- Roughness (particle children)
- New image, collection, text (in menus)
- Parents (particles)
- Wrap (text)
- Light (add menu)
- Empty (volume add menu)
- Empty (empty add menu)
- Cycles (f-curve modifier)
- Drag (workspace tool type)
- Power (light intensity)
- Power (math nodes)

This last change also moves all math operations in nodes to the
ID_nodetree context. It's needed only for some operations, but we
can't be more granular here.

Also...

- Fix context extraction for interpolation mode headers in F-Curves
  and GPencil interpolation operator
- Enable new translation: "Slot %d" in image editor
- Fix an English message in the node editor:
  "Replace the input image's alpha channels by..." -> channel

Ref. T43295

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D15694

source/blender/nodes/intern/node_util.c

@@ -200,7 +200,7 @@ void node_math_label(const bNodeTree *UNUSED(ntree), const bNode *node, char *la
   if (!enum_label) {
     name = "Unknown";
   }
-  BLI_strncpy(label, IFACE_(name), maxlen);
+  BLI_strncpy(label, CTX_IFACE_(BLT_I18NCONTEXT_ID_NODETREE, name), maxlen);
 }
 
 void node_vector_math_label(const bNodeTree *UNUSED(ntree),