Gradient fix part 2

Gradient Fix:

All gradients get imported correctly if SVG are grouped in path groups or not.
All gradients can be exported to SVG , only usespaceonuse gradientUnit is used for exporting.
Some property of gradients are deleted , new are inserted.
If you check the Gradient import test of the website all pass ok now, old import test still run fine.
A little change ( good change ) to rect class has been necessary and i included here.

Changes to polygon and polyline are necessary, but because i have to change test units with them, i will make a different PR for them.