Remove removeDecendentConstraints

[mattiasjahnke]

This PR removes removeDecendentConstraints method, since the method basically removed all constraints that involved its subviews (and not only constraints added by the OAStackView layout).

Changes:

• OAStackView will use the property for alignmentStrategy and distributionStrategy rather than the synthesized instance variable
• Implemented setAlignmentStrategy and setDistributionStrategy that removes the previous strategies constraints on OAStackView
• OAStackViewDistributionStategy now adds the constraints created by alignLastView: and alignFirstView: to its array of constraints
• Removed the removeDecendentConstraints method.
• OAStackView will only remove the strategies constraints during layoutArrangedViews

[mattiasjahnke]

I found some problems when testing this in the Example app that should be resolved now.

• The example storyboard had a bunch of constraints set on the buttons in the OAStackView. They were previously removed by the removeDecendentConstraints method but obviously caused some problems when I had removed it. I've set 'remove at build time' on these constraints.
• My previous implementation removed the strategies constraints by:

[self.distributionStrategy removeAddedConstraints];
[self.alignmentStrategy removeAddedConstraints];

This caused a layout to occur after the distributions removal and before the alignment - resulting in constraint warnings. I've therefore exposed the constraints on those objects as a readonly array so that OAStackView can remove all of them without causing a layout before all constraints are removed.

[delebedev]

@mattiasjahnke thanks for your work! please resolve final comments and I will merge PR

[mattiasjahnke]

Taken care of ;)

[delebedev]

Merged, thanks @mattiasjahnke!