Change selection outline decoration

The old selection outline draw method was implemented using the
technique of Bill Atkinson. This method has the disadvantage of not
looking like marching ants with selection borders that aren't parallel
to the coordinate axes.

The new method uses a QPen with a dash pattern to draw the ants, so they
are always displayed as dashes, no matter the angle that borders has.
It is draw in two passes. The first one uses a QPen configured
to draw the selection outline in white. Then, the selection outline is
draw again above the white outline, but this time using other QPen
configured to draw the ants in black.

Also, the outline is drawed without antialiasing.