Allow Download of Multiple Data with Fewer Clicks

Specification as follows:

The current download buttons should be re-used for the following features:

• User selects station: The download button tooltip changes to "Alle Werte der Station herunterladen" and allows downloading of all data of the station without additional selection.
• User selects station and a set of clients: The download button tooltip changes to "Alle Werte der ausgewählten Clients herunterladen" and allows downloading of all data of the selected clients.
• User selects station, a set of clients, and a set of sensors: The download button allows for download of the specific sensor data (just like it is now)