[feature request] inform of ongoing operations

When loading lots of data, the screen is updated several times.
We might want to inform the user to wait until all the data is ready.

Try for example
http://chadnorwood.com/gcm/?sd=2011-07-01&ed=2013-12-31&gc=aleaikido.com_dejam6fk5hftrvq9iu0gbar9bk@group.calendar.google.com,aikidorennes@gmail.com
The final map is on Western Europe, not just Brittany (Western France).
