Missing & incorrect Typescript type definitions

Here are some incorrect type definitions which I found:

InlineEditorMethods.setMapping(): all functions (init, onShow, onHide, destroy) are optional
DatastoreMethods.attachEvent & InlineEditorMethods.attachEvent should return a string

The following type definitions are missing (and in some cases also not documented):

ZoomMethods.getLevels()
```
getLevels(): ZoomLevel[] | undefined;
```

interface ZoomLevel { /**

the name of the level */ name: string;

/**
the height of the scale */ scale_height?: number;

/**
the height of the scale */ height?: number;

/**
the minimal width of a column. It has a higher priority than minColumnWidth and maxColumnWidth */ min_column_width?: number;

/**

an array of scales to switch between while zooming in/out on this level */ scales: Scale[]; }

- DomHelpers.locateAttribute()
```ts
locateAttribute(e: Event, qualifiedName: string): Element | null;

DomHelpers.getRelativeEventPosition() could return something like:
```
interface RelativeEventPosition{
x: number;
y: number;
}
```

DomHelpers.getNodePosition() could return something like:

interface NodePosition{
x: number;
y: number;
width: number;
height: number;
right: number;
bottom: number;
}

GanttStatic.getTaskPosition() could return something like:

interface TaskPosition {
left: number;
top: number;
height: number;
rowHeight?: number;
width: number;
}

GanttStatic.getState() could return something like:
```
export interface IGanttState {
/**
```

the id of a task that the user is currently dragging in the Gantt chart. 'Undefined' or 'null', if no tasks are being dragged in the Gantt chart. */ drag_id?: string | number | null;

/**
the drag mode. 'Undefined' or 'null', if no tasks are currently being dragged in the Gantt chart. */ drag_mode?: 'move' | 'resize' | 'progress' | 'ignore' | null;

/**
the flag for the fullscreen mode. 'True', if the Gantt chart is in the fullscreen mode, 'false' otherwise. */ fullscreen: boolean;

/**
the id of a task that is currently opened in the lightbox. 'Undefined' or 'null', if no tasks are opened in the lightbox. */ lightbox?: string | number | null;

/**
the date that tasks are displayed in the chart from */ min_date: Date;

/**
the date that tasks are displayed in the chart till */ max_date: Date;

/**
the id of the currently selected task. 'Undefined' or 'null', if no tasks are selected in the Gantt chart. */ selected_task?: string | number | null;

/**
the unit of the background grid of the timeline */ scale_unit: string;

/**

the step of the background grid of the timeline */ scale_step: number; }

- GanttConfigOptions.layout could use some actual values (a bit complex for this issue)
- GanttStatic.getScrollState() could return something like:
```ts
interface ScrollState {
x: number;
y: number;
}

DHTMLX / gantt

Missing & incorrect Typescript type definitions #102