client: compare surveys: prevent loading of differences from blocking the whole dialog The loading process of differences data is now visualized inside of the button to toggle between differences and the survey layer. The button is now disabled and says 'calculating differences' for the time of the calculation

teichmann@intevation.de = sascha.teichmann@intevation.de
bernhard.reiter@intevation.de = bernhard@intevation.de