Add check for integer in lengthInput

The message up top changes to '...must be an integer' if the value
inputted is either 1. not a number or 2. not an integer (i.e. doesn't
round evenly).

TODO: see if there is a better way to handle the "not an integer"
notification, including popup, tooltip, or maybe even just bringing
more focus to the changed text.
Change file indicator hover-over to random color

...by adding it to the list of styles modified by randomColorStyle.
This requires a special case where I convert the hex value to rgb and
attach an alpha of 0.5, since my current randomColorStyle setup doesn't
allow different cases (But who knows!)