Add ability to merge ajax callbacks when overriding ajax settings

[kartik-v]

Enhance ability to override beforeSend, success, complete, and error callbacks in ajaxSettings and ajaxDeleteSettings by allowing one to merge the callback functions with the default behavior.

Two new properties added:

1. mergeAjaxCallbacks: Applicable for override of ajaxSettings. Defaults to false. Can take a value of boolean false OR the string before and after.
   • if set to false, then the callback set in ajaxSettings will overwrite the default ajax callback.
   • if set to before then the callback set in ajaxSettings will be merged before the default ajax callback.
   • if set to after then the callback set in ajaxSettings will be merged after the default ajax callback.
2. mergeAjaxDeleteCallbacks: Same as above but applicable for ajaxDeleteSettings.

[kartik-v]

Resolved via updates to release v4.4.4.