Encode camera/layer state in URL

[josephbirkner]

Currently, Erdblick always resets to a default state and camera perspective upon reloading. This should be improved, such reloading never leads to a change of perspective or visible map layers.

The following information should be stored in the URL:

• Camera state information as needed by Cesium. It should be allowed to specify only a subset of this state, e.g. longitude and latitude, while the pitch/yaw/altitude assume default values.
• Active map layers and their respective display levels.

[MisterGC]

And in addition the set of activated styles (#76) and the OSM Layer setting (#27)