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* unitless events need be signified as such
* the event handler is now obligatory and must be a function
* keep the list of unit events on the frame itself
* workaround for bugs with RegisterUnitEvent
* postpone unit event registration for header units

Header units are either `party` or `raid`

UnitPower returns proper results for vehicles since patch 8.1

FrameXML\TargetFrame.lua uses UnitEffectiveLevel

None depend on spells currently and the tag funcs check for the right spec

Fixes #469

* Expose ColorGradient through the global oUF object and the unit frames

It was causing issues in scenarios where you'd override the original element but
didn't want to override the arena prep, or the other way around

Also minor fixes to documentation and additional comments for future reference