Implement custom delete confirmation dialog in Task Editor

[pmiklashevych]

Requested here: https://www.bryntum.com/forum/viewtopic.php?f=51&t=17474&p=86684#p86684

https://www.bryntum.com/docs/gantt/#Gantt/feature/TaskEdit#config-confirmDelete
We have an option to ON/OFF it. Need a way to configure it.

Think of adding async context to the beforeTaskDelete handler to be able to confirm and continue task removing.

Workaround for now:

const gantt = new Gantt({
    features : {
        taskEdit : {
            // Disable default confirmation
            confirmDelete : false
        }
    },

    listeners : {
        beforeTaskDelete({ editor }) {
            // Add custom confirmation dialog
            if (!editor.confirmed) {
                const autoClose = editor.autoClose;
                editor.autoClose = false;

                // Show dialog here
                MessageDialog.confirm({
                    title       : 'Sure to delete?',
                    message     : 'Press YES to delete!',
                    rootElement : gantt.features.taskEdit.rootElement
                }).then(result => {
                    editor.autoClose = autoClose;

                    if (result === MessageDialog.yesButton) {
                        editor.confirmed = true;
                        // `delete` is a private function, but that is how to get back to delete operation
                        gantt.features.taskEdit.delete();
                    }
                });

                return false;
            }

            delete editor.confirmed;
            return true;
        }
    },

[pmiklashevych]

Ideally the confirmation should be shown in all cases: editor, context menu, etc. There is a solution to add confirmation on beforeRemove on TaskStore, but there is a bug: #2878