Fix missing `particlemode` context mode, after recent particle-removal revert.

This caused the particle mode tools panel to appear in object mode, where it
misses some contextual data and spews python exceptions.

source/blender/blenkernel/intern/context.c

@@ -950,6 +950,7 @@ static const char *data_mode_strings[] = {
 	"weightpaint",
 	"vertexpaint",
 	"imagepaint",
+	"particlemode",
 	"objectmode",
 	NULL
 };