Default to not converting ScalarUnit limits

When ScalarUnit widgets do not have their hundred percent value properly initialized, converting between percentage and absolute units causes undesirable results, such as fixing the range to [100,100]%. In some instances, such as the scale tab in the transform panel, the input is active in circumstances where it's impossible to have a meaningful hundred percent value.

While the ideal solution would be to clean up all of these instances (e.g., make the scale inputs insensitive when there is no selection), the safer short term approach is to only enable the limit conversion behavior in known-good use cases, allowing remaining use cases to be addressed and tested individually.

Fixes #6287 (closed).