Categorized renderer fixes

@NathanW2 @nyalldawson  @leyan

Three fixes to categorized renderer

1) Fixed interaction between model reordering and link boundaries option so that link boundaries will sort before relinking
2) Changed label formatting to use QString.replace instead of QString.args to ensure that %1 and %2 are replaced by lower and upper bounds string respectively.  Also added tool tips and moved label format options down in GUI to reflect significance and position in grid below
3) Changed decimalPlaces to precision and allowed negative values as per Nyall's suggestion
This breaks a API.  Both methods will have to stay.