Bug 1039984 - [System] Trying to open a smartcollection after killing th...

...e Homescreen do nothing.

Allow Homescreen application window to be killed as other windows.
Agree, but the we need to split the responsibilities in the base class `AppWindow#kill()` method. As I found lots of special cases [`if (this.isHomescreen)`](https://github.com/lodr/gaia/blob/bug-1039984-smartcollections-and-homescreen/apps/system/js/app_window.js#L426), I choose this option.