minor changes - everythings is running again

DONE:
 - drawing is now organized by a own signal for all the gdv-elements
 - layer-contents have now some style-relevant private members
 - in principle the 2d-linear plotting is working fine

TODO:
 - linear axis is still not a own superclass; many things are at the moment
   written in the gdv_axis_class, that should be linear-axis specific
 - gdvaxis, gdvlayer and gdvcontent should be much better organized
 - element-class is still missing the option to interact with the element
 - documentation is widely still missing
 - a lot of axis and styles are not yet implemented