app: fix some oprtation descriptions and make them translatable

They are going to be visible in the UI.

app/operations/gimpoperationsemiflatten.c

@@ -33,6 +33,8 @@
 
 #include "gimpoperationsemiflatten.h"
 
+#include "gimp-intl.h"
+
 
 enum
 {
@@ -79,7 +81,7 @@ gimp_operation_semi_flatten_class_init (GimpOperationSemiFlattenClass *klass)
   gegl_operation_class_set_keys (operation_class,
                                  "name",        "gimp:semi-flatten",
                                  "categories",  "color",
-                                 "description", "Replace partial transparency with  a color",
+                                 "description", _("Replace partial transparency with a color"),
                                  NULL);
 
   operation_class->prepare = gimp_operation_semi_flatten_prepare;

app/operations/gimpoperationthresholdalpha.c

@@ -29,6 +29,8 @@
 
 #include "gimpoperationthresholdalpha.h"
 
+#include "gimp-intl.h"
+
 
 enum
 {
@@ -74,7 +76,9 @@ gimp_operation_threshold_alpha_class_init (GimpOperationThresholdAlphaClass *kla
   gegl_operation_class_set_keys (operation_class,
                                  "name",        "gimp:threshold-alpha",
                                  "categories",  "color",
-                                 "description", "Threshold a buffer's alpha channel to a value",
+                                 "description",
+                                 _("Make transparency all-or-nothing, by "
+                                   "thresholding the alpha channel to a value"),
                                  NULL);
 
   operation_class->prepare = gimp_operation_threshold_alpha_prepare;

po/POTFILES.in

@@ -236,6 +236,8 @@ app/operations/gimpcurvesconfig.c
 app/operations/gimplevelsconfig.c
 app/operations/gimpoperationcagecoefcalc.c
 app/operations/gimpoperationcagetransform.c
+app/operations/gimpoperationsemiflatten.c
+app/operations/gimpoperationthresholdalpha.c
 
 app/gui/gui.c
 app/gui/gui-message.c