VSI: Vertical Situation Indicator



GA pilots are often planning the altitude to keep between all the WPs in their route, for example:
![altitudegraph](https://f.cloud.github.com/assets/3354240/2304405/03a11502-a21c-11e3-953d-da36a56fb6fe.png)


This change allows to compare our current altitude with the one planned in the flight plan giving intuitive indications on a bar displayed in the HSI screen:
![lk8000-vsi](https://f.cloud.github.com/assets/3354240/2304406/0eec81b2-a21c-11e3-8a80-0aeffe54dd12.png)


In this example the VSI (Vertical Situation Indicator) bar it is telling that we are under our planned route of more than 300 ft and that the ground is just at 300 ft (100 m) below us.

VSI bar is displayed only if we have a task loaded and if the glide slope bar is not already shown.