Fix ValueError on interactive signal range select

When selecting a range by the SpanSelectorInSpectrum, a ValueError is
raised initially when the left value is assigned, and the right value is
yet to be assigned (since the left can be higher than the right value).
Avoided by initing to NaN, and comparing right value to NaN on a ValueError.

Also added Thumbs.db (a windows thumbnail cache file) to .gitignore.