Implement command for dragging

[david-tejada]

Depends on #107 and #108.

Sometimes the drag handle only appears as you hover over the element, so we probably need to dispatch the hover events first and then look for an element in the vicinity with cursor: grab. Once we have the elements coordinates and the target coordinates we can send the command with the origin and target coordinates to talon to drag the element.

I want the following commands to be possible:

• drag bat to drum
• drag bat below drum
• drag bat above drum
• drag bat before drum
• drag bat after drum

For the relative positions we would use the target as defined in #108.