FLUID-5190: Improved the text field slider component.

To get rid of the dependency on the hard-coded model path "value".

To be documented:

API of text field slider component has been modified to add one more component option "modelPath" for integrators to specified the model path for saving and retrieving the processed value, rather than the old way that requires this value being supplied in a fixed model path "value".

An example of how to use the new API in a wrapper component:
https://github.com/cindyli/infusion/blob/FLUID-5190/src/framework/preferences/js/Panels.js#L62

More examples in unit tests:
https://github.com/cindyli/infusion/blob/FLUID-5190/src/tests/component-tests/textfieldSlider/js/TextfieldSliderTests.js#L29