plug-ins: rename "Preset" to "Source type" in WebP export dialog.

The "Preset" feature changes encoding parameters not exposed in our GUI.
Thus it looks like the option is broken as whatever you select, nothing
is updated in the export dialog.
This is also inconsistent with what we call presets in the rest of GIMP:
they are used to save and load values for all fields as a dynamic and
editable list (whereas here that's a pre-defined list in libwebp setting
only a subset of options).

`cwebp` manual defines "preset" as: "Specify a set of pre-defined
parameters to suit a particular type of source material."

Thus call the field "Source type" and add a tooltip "WebP encoder preset"
so that people don't expect a way to save settings whereas the ones used
to the WebP encoder parameters still find their way.

plug-ins/file-webp/file-webp-dialog.c

@@ -135,7 +135,10 @@ save_dialog (WebPSaveParams *params,
                     &params->alpha_quality);
 
   /* Create the label for the selecting a preset */
-  preset_label = gtk_label_new (_("Preset:"));
+  preset_label = gtk_label_new (_("Source type:"));
+  gimp_help_set_help_data (preset_label,
+                           _("WebP encoder \"preset\""),
+                           NULL);
   gtk_label_set_xalign (GTK_LABEL (preset_label), 0.0);
   gtk_table_attach (GTK_TABLE (table), preset_label,
                     0, 1, 2, 3,