Slider: reset input on NaN values

When a value cast by Number to NaN is given to the slider input field,
reset the input to the current slider value (or the first value if the
previous value is also NaN).

Closes acardona/CATMAID#850.
Navigator: swap behavior of shift when +/- zooming

+/- now change zoom by major steps by default, and by minor steps with
Shift held. Behavior of using the scroll wheel over the slider is
unchanged, i.e., scrolling changes minor steps by default and major
steps with Shift held.

Closes acardona/CATMAID#851.