For historical reasons, they are stored in mils, but using int create rounding
issues when converting to/from mils to IU in dialogs.
This is a minor but annoying issue.
Fixes#16000https://gitlab.com/kicad/code/kicad/-/issues/16000
Also cleans up the dialog a bit.
Also changes the workings of NormalizePath to return the input if
it couldn't be shortened which saves open coding that behaviour in
all its callers.
Fixes https://gitlab.com/kicad/code/kicad/issues/9036