can't set stickyHeaders` in Storyboard for IGListCollectionViewLayout

[aelam]

New issue checklist

stickyHeaders is a readonly property and if we initialize it from XIB or storyboard. we have no chance to set it to YES or modify it.

- (instancetype)initWithCoder:(NSCoder *)aDecoder {    
   return [self initWithStickyHeaders:NO topContentInset:0 stretchToEdge:NO]; 
}

• [TRUE ] I have reviewed the README and documentation
• [ TRUE] I have searched existing issues and this is not a duplicate

General information

• IGListKit version:
• iOS version(s):
• CocoaPods/Carthage version:
• Xcode version:
• Devices/Simulators affected:
• Reproducible in the demo project? (Yes/No):
• Related issues:

Debug information

# Please include debug logs using the following lldb command:
po [IGListDebugger dump]

[rnystrom]

Ya, this is honestly just a limitation of storyboards. We could maybe make this a property editable from SB, tho it's my super high pri.

[rnystrom]

One recommendation is to just change the layout in code after things load.

[jessesquires]

I think we can mark this as IB_DESIGNABLE and it will show up in IB

[rnystrom]

I can't remember if there is extra work that the layout does in init with sticky headers. I don't think so. We can move this to a designable propane phase out the initializer. Plus we are gonna make breaking changes to the init in 4.0 anyways after the h scroll support.

Cools?