Check text widgets with numerical input, set appropriate increment when the up/down buttons are clicked

[edan-bainglass]

E.g., for the cell vector lengths, use 0.1 ang instead of 1 angstrom (too much)

[edan-bainglass]

While we're at it, we should also check that all min/max make sense. There are cases for example where a nonsensical 0 is permitted (or negative values even 😬)