Resolve "Implement placeholders for the config"

This adds placeholders to input fields that don't have default values and changes the validation error message when someone enters a different number of sample names and run numbers. Also fixed the following bugs:

1. When closing a data preview window, we would get a JSError due to some oddities with nested VMenus. To fix this, I've moved the ONCat selection menus to prevent this nesting. I had to also visually move them to the right of each text field in order to accommodate the code change (this is probably a positive change on its own, though).
2. The stitching inputs when multiple Q-ranges have been added had a bug where they would resize on focus.

Closes #26 (closed)