[changed] set the position of the menu as fixed

[CorentinDeBoisset]

This is a copy of the changes from #75 without the conflicts

[CMTegner]

Just a heads-up (in case you're waiting for this PR to land): You can fully customize the dropdown menu by using props.menuStyle and props.renderMenu.

We'll probably be removing most of the default menu styles for 2.0, possibly only keeping the absolute coordinates which are passed to props.renderMenu (although you could calculate this yourself if we added support for rendering custom components instead of just fragments for the menu).