Enable dragging dialog to a new position

[OlliTietavainenVaadin]

There should be a toggleable option for making the dialog draggable (repositioning by mouse drag and drop). Related issue: https://github.com/vaadin/vaadin-dialog/issues/120 is possibly needed first.

[jouni]

Notes for implementation:

Must have:

• The end user can reposition the dialog with mouse drag

Should have:

• setBounds(top, right, bottom, left) API (this could be internal at first, low-level API, and we can decide if it makes sense to expose it as public)
• center()

Details:

• Resizing (Enable resizing the dialog #132) takes priority over repositioning
• The dialog should start using absolute positioning (and “snap to pixels”) once the users starts to drag it (instead of flexbox based centering)
• The whole dialog container (part="overlay") acts as the draggable surface. Any content inside the dialog, which does not specify pointer-events: none will block the drag.
• The user should not be able to drag the dialog completely outside the browser viewport. A certain minimum area of the dialog should always stay visible.

API:

• <vaadin-dialog draggable>
• vaadinDialog.draggable = true

[web-padawan]

Fixed by #139