Added extra navigation symbols.
Initial implementation of pixel perfect chart symbols. Adds symbols for
different rock types and left and right buoys.

Created data structure that contains the type of the
point object received from the database. Extended ChartObjectInterface
and classed Rock and NavaId to support the data structure.

Disabled QGraphicsItem's cache mode in ChartWidget. When chart item size grow
over a certain limit the caching behavior became slower than drawing the
item again using the item's paint method. Disabling caching results that
the chart item is re-drawn constantly, but the performance stays constant with
every zoom level. More elaborate means to handle the caching is needed
if better performance is required at later date.