Componentize things

I've refactored many parts of the app into components.

* The application menu is a component called `<slide-menu>`. It captures touch gestures and react to them.
* The semitransparent black overlay is a component named `<content-overlay>`. It very simple, and is sinchronyzed with the progress of the lateral menu.
* The animated button in the top bar that toggles the menu is another component `<toggle-menu>`, is also synchronized with the menu.

Te result is that all animations are synchronous. The effect is very nice.