Decouple Layer Placement Strategy From Core

[weissi]

The idea is to decouple the layer placement (and re-placement) strategy from core FRLayeredNavigationController. This would allow the implementation of e.g. of dynamic placing strategies like

• make every layer at least x points wide, rearrange all layers if space is not available (sample implementation directly integrated into the FRLayeredNavigationController core: mronkko/FRLayeredNavigationController@f0109c7 )
• exponential spacing (very old implementation when this was FRLayeredNavigationController's (called FancyVC back then) default strategy: c92b0f8)
• ...

The work to do would be:

• define an sensible API for different kinds of layer placement strategies
• write tests for the current placement strategy
• decouple the current strategy to a class that FRLayeredNavigationController uses by default (when it's not provided another placement strategy)

The prerequisites would be (and therefore best implemented along with Issue #5 and Issue #13):

• unit tests
• probably an explicit data model for FRLayeredNavigationController