revert multi-line support for custom block text slots

because the don’t support brown dots for spaces

blocks.js

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