Slider: if rage option is true, the first handle value can't exceed the second one. Fixed #3762 - slider handles not restricted properly when set programmatically

By now, if range option is set to true, only two handles are supported.
So, I created a function _verifyValues() and invoked it inside
_createRange() and values( index, newValue ) whenever necessary. Also, I
created a unit test.
Slider: Avoid the slider to be locked when both values are max (if range option is true). Fixed #9046 - Setting both values of range slider to maximum value, locks the slider

Basically, by the time both handles are set to the maximum value, I give
a focus on the first handle. Doing that, the lock is avoided because the
user will have control on it (first handle), allowing drag it to the
left whenever necessary. I also added a visual test.
Slider: Mix, Max & Step is now being forced into numeric values. Fixed #8144 - Min, Max & Step not being forced into numeric values