Fixed and improvements in the Triangle class.

Were removed two typos that, naturally, raised problems at compiling
time.

Triangle's constructor changed and now receives const references not
pointers.

Default colors for Triangle's inner area and border are defined at
construction time.
A GraphicsItem is now renderable.

All vertices coordinates of a GraphicsItem are now loaded from a file
via the loadFromFile method. Due time concerns this method is still very
simple and has no treatments for malformed input files.
GLView modified to test GraphicsItem's new features.

A test file is given as input to GraphicsItem::loadFromFile() and its
draw() method is called each paintGL() event.