[FEAT] Render Task

[csouchet]

Is your feature request related to a problem? Please describe.
Render the Task with the BPMN standard.

Describe the solution you'd like
Create a new MxGraph Shape.

BPMN specification
A Task object shares the same shape as the Sub-Process, which is a rectangle that has rounded corners.
A Task is a rounded corner rectangle that MUST be drawn with a single thin line.
The use of text, color, size, and lines for a Task MUST follow the rules defined in “Use of Text, Color, Size, and Lines in a Diagram” on page 39. (See in issue #136)

• A boundary drawn with a thick line SHALL be reserved for Call Activity (Global Tasks) (see page 186).
• A boundary drawn with a dotted line SHALL be reserved for Event Sub-Processes (see page 174) and thus are not allowed for Tasks.
• A boundary drawn with a double line SHALL be reserved for Transaction Sub-Processes (see page 176) and thus are not allowed for Tasks.