Integration with Subtaskdate plugin to display subtask deadlines into Gantt plugin

[gillesgamech-cyber]

I am a heavy user of the Gantt plugin (developed by @fguillot) and I find it essential for project oversight. However, for complex projects, many users also rely on the [Subtaskdate plugin] to assign specific start and end dates to subtasks.

Currently, these subtask milestones are invisible in the Gantt view, which only displays the main task deadlines. This makes it difficult to visualize the internal schedule of a task.

ince the Gantt plugin is a core component of the Kanboard experience, I suggest adding support for Subtaskdate fields. Specifically withe a visual Hierarchy: display subtasks as child elements (indented or nested) under their parent task within the Gantt chart.

Thank you for your incredible work on Kanboard !