gr_basic.cpp now clips filled polygons,using the Sutherland and Hodgman algo to clip the poly against a rectangle
Filled polygons are now correctly displayed under Linux(i hope)

(does not delete segments or does not deleting areas outlines)

Dialog track options redesigned with wxFormBuilder Alt via drill renamed Specific via drill. Added tips in dialog.

Prevent drawing center from moving when client zoom size exceeds drawing size.
Prevent drawing refresh when zoom level doesn't change to eliminate flicker.