[IMP] qweb: add preliminary support for transitions

Still very naive...

Part of #66

doc/animations.md

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+# 🦉 Animations 🦉
+
+
+Animation is a complex topic. There are many different use cases, and many
+solutions and technologies.
+
+## Simple CSS effects
+
+Sometimes, using pure CSS is enough.  For these use cases, Owl is not really
+necessary: it just needs to render a DOM element with a specific class. For
+example:
+
+
+```xml
+<button class="flash" t-on-click="doSomething">Click</button>
+```
+
+with the following CSS:
+
+```css
+.flash {
+  transition: background 0.5s;
+}
+
+.flash:active {
+  background-color: #41454a;
+  transition: background 0s;
+}
+```
+
+will produce a nice flash effect whenever the user click (or activate with the
+keyboard) the button.
+
+## CSS Transitions (single element)
+
+A more complex situation occurs when we want to transition an element in or out
+of the page.  For example, we may want a fade-in and fade-out effect.
+
+The `t-transition` directive is here to help us (see [QWeb documentation](doc/qweb.md#t-transition-directive)).