Bigger drop area for file imports

[Ben Wiley]

• onDragEnter handler for SampleRecord component that displays a big overlay across the whole component, for dropping files
• onDragLeave handler for SampleRecord component that hides the overlay
• Possible onDragOver handler for SampleRecord component to prevent default ?
• onDrop handler for SampleRecord to call the file input handler

All these handlers are disabled if thecaptureState is 'capturing' or 'finalizing'.