Add gridline customization

[matoos32]

Implements enhancement #64:

• Adds configuration of major gridlines.
• Adds configuration of minor gridlines.
• Deprecates gridLines() method for removal.
• (Breaking change) Removes gridLines() noGridLines() showGridLines() from XYTimeSeriesPlotBuilderElements but risk is low as this is an internal class.
• By default major gridlines are now configured ON with default styling.
• Updates demo app with config class for various scenarios and new defaults cases.
• Extracts a few convenience color constants to BuilderConstants.

[matoos32]

Code looks good. All findings resolved. Tested on Windows 10 using JDK 11. No issues observed with this change set.