Based on patch from @lbud (Lauren Budorick).
Give `HeatmapColorPropertyValue` a more generic name,
since the same value type will be used for both `heatmap-color`
and `line-gradient` properties.
* [ios, macos] new struct MGLTransition
* [ios] MGLTransition struct moved to MGLTypes.h
* [ios] MGLTransition implemented for style layer properties
* [ios, macos] added support for getting property transitions
* [ios, macos] updated to MGLDurationFromTimeInterval and MGLTimeIntervalFromDuration
* [ios, macos] added transition properties test
* [ios, macos] modified key strings imp
* [ios, macos] The impl for transition properties were changed to public methods
* [ios, macos] Changelog update
* [ios] Fixed layer space formatting
* [core] The name space was fixed for getters that returned TransitionOptions
* [ios, macos] Added rawLayer transition property tests
* [ios] Added a factory method for MGLTransition and transition-related methods to NSValue(MGLAdditions)
* [ios, macos] Copyedited changelog blurbs about transition properties
* Layer source ID is immutable; must be provided to the constructor
* Layer source layer is mutable
* Layers with GeoJSON sources do not have a source layer
While here, make Layer::copy impl-private.
