"Layer Speed" should be displayed as "Float" type value (6-digits limit) instead of double

[LoveBodhi]

[Shadowblitz16]

Unless the format of the speed changes it's nessary. It's still not accurate enough to do some speeds and delays

[Wohlstand]

Actually, the number of precision depends on how large the whole part is. Internally this is a "double" type, and I had to set larger number of digits precision since it's a limitation of Qt's float spin box that doesn't supports variadic precision. In the past I already had the smaller precision, but that led serious data loss. That why I set larger one.