[feature request] inform of ongoing operations

[igorrosenberg]

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).