Currently, if the client's sidebar is opened it doesn't behave
graciously when the browser window is resized. Sometimes the sidebar
appears floating on the middle of the window.

We discussed several alternative solutions: when the sidebar is opened
and the window is resized, then...

1. close the sidebar and the user opens it manually
2. maintain the sidebar open, but scale it properly
3. close the sidebar and open it after X milliseconds of the last resize
   event
4. close the sidebar if the width of the window is reduced, but
   maintain it open if the window's width is increased

We implemented solution #2, because it gives a more consistent behaviour
for both sidebars, PDF and non-PDF.

In addition, I added a mechanism to register and unregister events, to
avoid resource leaks.