BoundsEditor: Slidebar cannot be adjusted when max value slider overlaps with min value slider

Code:

```python
from traits.trait_types import Range, Float
from traits.has_traits import HasTraits
from traitsui.api import Item, View
from traitsui.qt4.extra.bounds_editor import BoundsEditor

class Test(HasTraits):
valueFilter = Range(5.0, 10.0)

min = Float(5.0)
max = Float(10.0)

view = View(
Item('valueFilter',
editor=BoundsEditor(
low_name='min',
high_name='max',
format='%.2f')
)
)

Test().configure_traits()

```

### **In BoundsEditor, moving the max value slider to the complete left (overlapping with min value slider) doesn't allow the sliders to move anymore.**