#297 - Fix transparent resource dropdown.

The value of the CSS attribute "overflow" was set to "visible" (the
correct value) for the enhanced_dropdown class. Then, in the same file
(dropdowns.scss), the value was changed to "hidden", which is the wrong
value.

Chrome behaved differently from Firefox in this case (unknown reason).
Merge pull request #638 from arvindsv/fix_transparent_resource_dropdown

#297 - Fix transparent resource dropdown.