Clean up code: Easier to understand/maintain.

Also:
Reduce line count for code.
Add namespace (module) to use shorter function/variable names.
Improve auto-generation of readers/writers for arbitrary structs.
Remove a few manually-written readers/writers for simple structs.
Write Plots.jl version -> VERSION_INFO to help user identify reader of outdated file formats.
Add support for:
 - Arrow
 - ColorScheme, ColorPalette
 - ContinuousColorGradient, CategoricalColorGradient
 - datetimeformatter

src/backends.jl

@@ -665,11 +665,6 @@ mutable struct HDF5Plot_PlotRef
 end
 const HDF5PLOT_PLOTREF = HDF5Plot_PlotRef(nothing)
 
-#Simple sub-structures that can just be written out using _hdf5plot_gwritefields:
-const HDF5PLOT_SIMPLESUBSTRUCT = Union{Font, BoundingBox,
-	GridLayout, RootLayout, ColorGradient, SeriesAnnotations, PlotText,
-	Shape,
-}
 
 # ------------------------------------------------------------------------------
 # inspectdr