Relation tags support line styles

[kirchsth]

Relation tags uses basically the arrow color skinparams. The (new) arrow color supports line styles like below too

@startuml
class foo
foo --> bar : normal
foo --> bar1 #line:red;line.bold;text:red  : red bold
foo --> bar2 #green;line.dashed;text:green : green dashed
foo --> bar3 #blue;line.dotted;text:blue   : blue dotted
@enduml

Based on that added relation tags could support line styles too like

AddRelTag("backup", $textColor="darkgray", $lineColor="darkgray", $lineStyle = DashedLine())

[Potherca]

Fixed in #160