Implement "gray" when uicontrol pushbuttons/togglebuttons are disabled (bug #57128) * BaseControl.cc (updatePalette): For pushbuttons & togglebuttons, add ":enabled" to beginning of StyleSheet specification so that custom colors are only applied when widget is active. * BaseControl.cc (init): Call blockSignals() if "Enable" property is "inactive" during setup of widget. Otherwise, call setEnabled() with value of "Enable" property. * BaseControl.cc (update): When updating "Enable" property, call blockSignals() if value is "inactive", otherwise call setEnabled().