Add ability to customizing import/export process

[m0r1arty]

Examples: for process custom price list xlsx and/or customizing order item of fields/properties

[m0r1arty]

JS bug in @app/modules/image/widgets/ImageDropzone.php current code make JS part of page: "previewsContainer":"#w7","url":["upload"],"paramName":"file"}); then Dropzone.uploadFiles call XMLHttpRequest.open with url === ["upload"] but in 'debug' extension:

   XMLHttpRequest.prototype.open = function (method, url, async, user, pass) {
        var self = this;
        /* prevent logging AJAX calls to static and inline files, like templates */
        if (url.substr(0, 1) === '/' && !url.match(new RegExp("{{ excluded_ajax_paths }}"))) {

raise Error "TypeError: url.substr is not a function"

[bethrezen]

As dropzone bug does not relate to current pull request (import/export) - please split it to another PR.

[m0r1arty]

Done?