#1 bug fixes and implementing ZOrder

[Hernan Foffani]

I'm posting here some patches that allow some kind of
ZOrder for Shapes.
With it, it's possible to place different shapes on top of
others hiding these.
For minimal source change, I used the "shapes"
collection. This way the only thing I needed was a
a method to push a shape to the back.

Besides, I made a change to deselect a shape by
clicking over it while pressing the shift key.

There are also, a couple of possible bugfixes:
- In NetronPanel.cs all the if sentence of "TODO:
Automatic layout" should be inside the true block of the
if (c.AcceptConnection ... etc.
It may fail if a user overrides this method returning false
sometimes.

- In Connection.cs:Invalidate() before Inflating you
should check if the Tracker is not null.

[Hernan Foffani]

patches for Netron/Core/*.cs

[Hernan Foffani]

patch for Netron/UI/NetronPanel.cs