Update design for editor tabs with icons, changing blocks to code.

Resolves https://github.com/LLK/scratch-gui/issues/1713

src/components/gui/icon--code.svg

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="20px" height="20px" viewBox="0 0 20 20" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <!-- Generator: Sketch 49.2 (51160) - http://www.bohemiancoding.com/sketch -->
+    <title>Code V2</title>
+    <desc>Created with Sketch.</desc>
+    <defs></defs>
+    <g id="Code-V2" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
+        <path d="M15,14.51 C15,14.786 14.776,15.01 14.5,15.01 L9.197,15.01 C9.064,15.01 8.937,15.062 8.844,15.156 L8.146,15.854 C8.053,15.947 7.926,16 7.793,16 L6.207,16 C6.074,16 5.947,15.947 5.854,15.854 L5.156,15.156 C5.062,15.062 4.936,15.01 4.803,15.01 L3.5,15.01 C3.224,15.01 3,14.786 3,14.51 L3,11.5 C3,11.224 3.224,11 3.5,11 L4.793,11 C4.926,11 5.053,11.053 5.146,11.146 L5.854,11.854 C5.947,11.947 6.074,12 6.207,12 L7.793,12 C7.926,12 8.053,11.947 8.146,11.854 L8.854,11.146 C8.947,11.053 9.074,11 9.207,11 L14.5,11 C14.776,11 15,11.224 15,11.5 L15,14.51 Z M17,9.51 C17,9.786 16.776,10.01 16.5,10.01 L9.197,10.01 C9.064,10.01 8.937,10.062 8.844,10.156 L8.146,10.854 C8.053,10.947 7.926,11 7.793,11 L6.207,11 C6.074,11 5.947,10.947 5.854,10.854 L5.156,10.156 C5.062,10.062 4.936,10.01 4.803,10.01 L3.5,10.01 C3.224,10.01 3,9.786 3,9.51 L3,6.5 C3,6.224 3.224,6 3.5,6 L4.793,6 C4.926,6 5.053,6.053 5.146,6.146 L5.854,6.854 C5.947,6.947 6.074,7 6.207,7 L7.793,7 C7.926,7 8.053,6.947 8.146,6.854 L8.854,6.146 C8.947,6.053 9.074,6 9.207,6 L16.5,6 C16.776,6 17,6.224 17,6.5 L17,9.51 Z" id="Code" fill="#4C97FF"></path>
+    </g>
+</svg>