[GANTT] Implement arrow style color

[Nordes]

Adding the arrow coloring as what we would expect. If the style exists or the default color is RED_DARK, then we try to get the style from the <style>...</style> else we use the style from the text in the puml.

Usage

<style>
@startgantt gantt
!$PRIMARY_LIGHT = "#33b2e2"
!$PRIMARY = "#009fdb"
!$DARK = "#5a5a5a"
!$SECONDARY_DARK = "#c2c2c2"

<style>
ganttDiagram {
	task {
		FontName "Cascadia Code PL"
		FontSize 11
		FontStyle bold
		FontColor $DARK
		BackGroundColor $PRIMARY_LIGHT-$PRIMARY
		LineColor $SECONDARY_DARK
	}

	arrow {
		LineColor #000000
	}
}
</style>
printscale weekly

Project starts the 1st of january 2021
2021-01-04 to 2021-01-10 are colored in #d2d2d2

[Automatic branch tag on release] as [s1] starts 2021-01-15 and lasts 1 week
then [Setup Automatic Alerts] as [s2] lasts 1 week

[End] happens 150 days after start
@endgantt

Result

fix #437.

[arnaudroques]

Many thanks for your contribution !
We will also remove this HColorUtils.RED_DARK hard coded value, which comes from first versions, when Style was not implemented in Gantt.

Thanks again!