Dropdown positioning issues

[cburgmer]

With the current solution

• Scrolling of inner elements (overflow: auto) will keep the dropdown fixed as it is attached to body. The visual effect is that the dropdown separates from the widget.
• Pages with extensive z-index usage will have issues, as the dropdown adjusts to the body.

This commit fixes these issues by attaching the dropdown to the same parent as the token-input.

Placing the dropdown with the parent has the downside of interfering with CSS overflow settings.

[cburgmer]

See #93 for a bug with positioning

[mrbrdo]

The fix worked for me, although the style is a little bit different than before. Tokeninput stopped working correctly as soon as I started using jQuery UI, but this fixed it. Would be nice if this could be merged!