Configurable conversion classes between Bootstrap Versions

[kartik-v]

Enhancements to allow conversions of CSS across bootstrap versions and auto detection of CSS class based on Bootstrap library version (currently 3.x and 4.x).

• BootstrapInterface includes all constants for various Bootstrap CSS Styles across versions prefixed by BS_
• BootstrapTrait includes new static property bsCssMap that will be used by all Krajee Bootstrap widgets and input widgets and will implement the BootstrapInterface as well.
• New BootstrapTrait methods getCssClass, addCssClass, and removeCssClass will help detect or add or remove the right CSS class for the specific Bootstrap Library Version