Add isHorizontal=true to Participant / Lane DI shapes

[nikku]

Is your feature request related to a problem? Please describe.

The isHorizontal attribute on participant and lane DI shapes is meta-data that helps editors to understand how these elements are laid out. Quoting from the BPMN spec (p. 373):

• isHorizontal : Boolean [0..1]
  an optional attribute that should be used only for Pools and Lanes. It determines if it should be depicted horizontally (true) or vertically (false).

The attribute is not necessarily required, thus this feature request falls into the forgiveness / compatibility category.

Describe the solution you'd like

We should add the attributes, as we only support horizontal modeling anyway and "fix" existing diagrams by gradually introducing the

• add isHorizontal=true on newly created participant / lane DI shapes
• add isHorizontal=true on update participant / lane DI shapes

Describe alternatives you've considered

• ignore compatibility concerns and keep things as is

Additional context

Reported in camunda/camunda-modeler#1096, too.

[barmac]

Closed via #936