Bugfix: positioning of dropzone

switch oc-dropzone positioning from fixed to absolute, fixed always orientates by the viewport which means it always covers the entire screen.
This is something we can't know and the decision should be left to the consuming app.

Instead we use position absolute to just cover the next parent which ist positioned relative to the viewport.
In situations where the body is higher or wider than the viewport fixed works better because absolute won't cover the scrollable parts of the content.

Since oc apps always do scrolling on their own this is no problem


https://github.com/owncloud/ocis/issues/1052