Allow text input on slider widgets

This adds "click-to-edit" to the text readout to the right of a slider, allowing precise values to be set.

Changes are entirely on the javascript. Editing is handled using the HTML5 `contentEditable` attribute rather than explicitly creating any input widget. The modified value is committed by either clicking outside the text, or pressing enter. Works for both `Int` and `FloatSliderView`.

The new value is validated for float/int and min/max, but the step size (if set) is not enforced.

This will need updating if #6050 is merged.
Would you object to renaming this to `_parse_value` so it's consistent with `IntTextView`?  I like the way you've declared it, it looks cleaner than the way I did which takes 3 lines:
```javascript
        _parse_value: function(value) {
            // Parse the value stored in a string.
            return  parseInt(value);
        },
```
While you're in this area of the codebase, feel free to change this in `IntTextView` and `FloatTextView` so it looks like yours:
```js
_parse_value: parseInt,
```