feat: add manual drag event handler

[pcurrivan]

Resolves #86

Adds an alternative event-based way to initiate dragging. This provides a way around the limitations of the current dragging options, namely:

• It is not possible to use the ktdGridDragHandler directive on elements in a child component, because ContentChildren cannot see inside child components.
• It is not possible to customize what pointer events should be used for dragging. For example, the new method would let a user choose to use middle mouse clicks to initiate dragging.

Adds a public method, onManualDragStart, to the grid-item component. This method accepts pointer down events, and uses them to initiate dragging, ~regardless of the draggable setting. Added comments recommending setting draggable to false if using manual drag events. For example:~

~<ktd-grid-item #gridItem [draggable]="false"> <my-item (myDragHandleMouseDownEvents)="gridItem.onManualDragStart($event)"></my-item> </ktd-grid-item>~

[pcurrivan]

I realized a couple of things are missing from this:

TODO:

• [x] Do not ignore draggable setting. Instead use a flag to switch to manual drag events mode. Update comments to reflect this.
• [x] Update README file with new API

[pcurrivan]

This should be ready.

[llorenspujol]

Thanks for you PR @pcurrivan !

I agree on the solution, I just would change some small details!

Let me know what you think, I added my suggestions on the issue: https://github.com/katoid/angular-grid-layout/issues/86#issuecomment-1563031368

[pcurrivan]

@llorenspujol

I responded to your comment https://github.com/katoid/angular-grid-layout/issues/86#issuecomment-1563095780

[llorenspujol]

Merged!! 👏