Allow Tasks to Be Editable - per task/bar - fixes #323

[lucfranken]

In follow up on: #323 (comment)

Allows to set the readonly flags in the tasks array. Which allows to turn on/off the editing of dates and progress per task.

Example in the README:

It is shown as a proof of concept but works.

Currently it works by just not showing the handles to edit the dates. Do you like to further secure those modifications in for example the event handlers? For example I do not see a check here for this.gantt.options.readonly_progress but only for the generic read-only flag.

    update_progressbar_position() {
        if (this.invalid || this.gantt.options.readonly) return;
        this.$bar_progress.setAttribute('x', this.$bar.getX());

        this.$bar_progress.setAttribute(
            'width',
            this.calculate_progress_width(),
        );
    }

Is that intentional or just an omission in the checks to make them exhaustive?