Increase "Length Step" (tlenstep) precision

[Lumikeiju]

Rationale
1 ft = 0.3048 m, but when the value 0.3048 is entered in the building_tools plugin's BuildingSizeDialog "Length step" (tlenstep) field, it is rounded to 0.305 and this causes a compounding error resulting in lengths like 100.07 ft when 100 ft is intended.

Fix
Use setMaximumFractionDigits() to support 4 decimal points:

    private final JFormattedTextField tlenstep = new JFormattedTextField(NumberFormat.getInstance());
    {
        ((NumberFormat) tlenstep.getFormatter()).setMaximumFractionDigits(4);
    }

in https://josm.openstreetmap.de/osmsvn/applications/editors/josm/plugins/buildings_tools/src/org/openstreetmap/josm/plugins/buildings_tools/BuildingSizeDialog.java

[Lumikeiju]

Updated BuildingSizeDialog.java for building_tools plugin: support 4 decimal places