Fixed several bugs in KisPerspectiveGridManager

What is done:
1) Now the manager doesn't even try to paint the lines inside the
polygon that is not convex. It used to lead to a really weird
picture of a hedgehog on a user's screen :P
2) Fixed an "infinity"-bug. Eigen::Hyperplane::intersection() returns
a random point in case the lines are parallel. Now an additional
check returns a point somewhere around infinity to workaround this
behavior.
3) Changed the algorithm that chooses a line to chop up into
"subdivisions". Now the farthest line from the focus is used for
the division.

Still to be done:
- Make KisPerspectiveGridTool interactive. It will be a really short
fix. I just need to overcome a small infinite loop somewhere ;)