Initial async work

Operations can now:
1) return their progress directly.
2) throw an error.
3) (ADDED) return a promise:
  + that resolves to their progress.
  + that rejects an error message (like throwing but asynchronous).
  For an example see the new operation "Wait" (Flow Control)

Added a flow control operation "Wait", which waits for the number of
milliseconds passed in as its argument. It is a fairly useless operation
but it does demonstrate how asynchronous operations now work.

A recipe like:
```
Fork
Wait (1000ms)
```
will only wait for 1000ms (each wait runs at the same time as each
other).

I have not looked into performance implications yet, also this code is
probably more complicated than it needs to be (would love help on this).

src/css/structure/layout.css

@@ -428,3 +428,35 @@ span.btn img {
     border-top: none;
     margin-top: 0;
 }
+
+@-moz-keyframes spinner {
+    from { -moz-transform: rotate(0deg); }
+    to { -moz-transform: rotate(359deg); }
+}
+@-webkit-keyframes spinner {
+    from { -webkit-transform: rotate(0deg); }
+    to { -webkit-transform: rotate(359deg); }
+}
+@keyframes spinner {
+    from {transform:rotate(0deg);}
+    to {transform:rotate(359deg);}
+}
+
+.loading-icon::before {
+    content: "\21bb";
+}
+
+.loading-icon {
+    -webkit-animation-name: spinner;
+    -webkit-animation-duration: 1000ms;
+    -webkit-animation-iteration-count: infinite;
+    -webkit-animation-timing-function: linear;
+    -moz-animation-name: spinner;
+    -moz-animation-duration: 1000ms;
+    -moz-animation-iteration-count: infinite;
+    -moz-animation-timing-function: linear;
+    -ms-animation-name: spinner;
+    -ms-animation-duration: 1000ms;
+    -ms-animation-iteration-count: infinite;
+    -ms-animation-timing-function: linear;
+}