multi-line support for text-type input slots in custom blocks

however, multi-line inputs do not support the brownish dots for space
characters.

blocks.js

@@ -857,7 +857,8 @@ SyntaxElementMorph.prototype.labelPart = function (spec) {
             part.isUnevaluated = true;
             break;
         case '%txt':
-            part = new InputSlotMorph();
+            // part = new InputSlotMorph(); // supports whitespace dots
+            part = new TextSlotMorph(); // multi-line, no whitespace dots
             part.minWidth = part.height() * 1.7; // "landscape"
             part.fixLayout();
             break;