ref #135 Navigation/history

* Adds a history for cgsee navigation. 
* Supports all implemented manipulators and static presets. 
* Adds a gui widget to visualize all saved steps with a small thumbnail.
i further prefer to return on empty or do stuff:
```
if(isEmpty())
   return;

... // do stuff
```
this saves you some indentation and improves readability of the code since it clearly separates the functions purpose from cases/states that are not supported