Finish integration of Flex Layout System

[migueldeicaza]

Currently at best, we have LayoutSubviews, but there is no support for anchoring, springs or any other layout system.

Should consider what options we have.

Perhaps Views could have a LayoutManager in charge of laying things out. Would be desirable to have a layout manager that can work by default with Add, and the question is whether attributes of how a view should be laid out should live in the view itself, or should be an attached property when they are added to the parent.

It seems that with layout managers, we could have general purpose pones like Grid, or Flex, and convenience View subclasses like HBox/VBox to finetune the layout.

[migueldeicaza]

I now have a branch with the Flex engine from Laurent Sansonetti and Stephane Delcroix here, but I have not had a chance to try it out yet:

https://github.com/migueldeicaza/gui.cs/tree/layout

https://github.com/migueldeicaza/gui.cs/blob/layout/Terminal.Gui/Flex.cs

Samples in the original C codebase:

https://github.com/xamarin/flex/blob/master/demo/FlexDemo/FlexView.m

[migueldeicaza]

New branch for layout work is flex

[migueldeicaza]

Ok, my current thinking is that I should perhaps introduce Views that can do layout, and we would have a FlexView, which would apply Flex rules to the children, and have "attached" properties on the views to configure the item properties.

While Flex is very good, it might be premature to make this the sole way to lay out objects.

[migueldeicaza]

Used an alternative system, might revisit flex layer