Fix validator for floats in Machine Settings dialog

The DoubleValidator depends on the system locale, requiring users with certain locales to enter floats with a comma instead of a dot (though confusingly floats are always represented with a decimal a dot). Instead of configuring the DoubleValidator with a locale that only accepts decimal dots, this commit uses a RegExpValidator, like other numeric fields in Cura does.

plugins/MachineSettingsAction/MachineSettingsExtruderTab.qml

@@ -137,6 +137,7 @@ Item
                 labelWidth: base.labelWidth
                 controlWidth: base.controlWidth
                 unitText: ""
+                decimals: 0
                 forceUpdateOnChangeFunction: forceUpdateFunction
             }
         }