Review UX of file browser

Fix #12 (closed)

Summary of changes:

• Replace resize: horizontal with react-reflex, and style resize handle.
• Condense directory listing (smaller padding and font-size, prevent wrapping, add ellipsis, etc.)
• Move items count and refresh button into status bar at the bottom.
• Remove buttons to refresh and go back to parent directory from current path input (to see more of the current path), and add ellipsis to input.
• Fix tooltips on current path input and directory items buttons, and tweak their timing (slower to show, faster to hide).
• Add scrollbar-width: thin to get a nicer-looking scrollbar.
• Improve design of loading state.