Add "Global.cs" to gather literial constants.
Add two interfaces to make type-checking stronger.
Extract frame validation logics into "FrameValidator.cs".
Convert high-layer frames from struct to class.
Merge branch 'ZhangShuai'

Conflicts:
	BIT.PilotlessPlane.Models/FrameA.cs
	BIT.PilotlessPlane.Models/FrameB.cs
	BIT.PilotlessPlane.Models/FrameC.cs