Refactor 'js-exec' config entries with JS events

[herrvigg]

Deprecate Refactor the integration logic with js-exec in the JSON config and prevent embedded JS code in JSON by deprecating javascript, see #1000. This simplifies qTranslate, improves security and the integration framework.
Events are now triggered from the core module during JS execution. This also gives a much better control of the loading sequence.

Execution

New events named 'qtxLoadAdmin:[page-key]' are now triggered from the core module on document. The main qtx object is passed as parameter.

Example of handler: $(document).on('qtxLoadAdmin:post', function( event, qtx ) { /*...*/ } );.

The page keys come from the admin-config entry in the i18n-config.json. Only the active keys for the page currently visited are passed, they could be several keys corresponding to different page selectors.

There must be some pages or post_type selector making the current page specific for the event to be fired. Entries without specific selector are irrelevant since they would apply to all pages.

More detailed examples:

post

// i18n-config.json
"admin-config": {
    "post": {   // <--- page key
      "pages": {
        "post.php": "",
        "post-new.php": ""
      },
    // "js-exec" --> obsolete, remove this entry

// JS handler for exec (ES5)
$(document).on('qtxLoadAdmin:post', function(event, qtx) {
   // do exec for this entry...
});

nav-menus

// i18n-config.json
"admin-config": {
    "nav-menus": { // <--- page key
      "pages": {
        "nav-menus.php": "action=edit|menu=\\d+|^$"
      },
    // "js-exec" --> obsolete, remove this entry

// JS handler for exec (with ES6 arrow function)
$(document).on('qtxLoadAdmin:nav-menus', (event, qtx) => {
   // do exec for this entry...
});

Loading scripts

Custom scripts should not be loaded by qTranslate anymore. The third-party plugins or themes should handle this, there could be some specificities that are not part of qTranslate and these scripts don't belong to qTranslate. They should be loaded in the footer. The only constraint is the dependency to qtranslate-admin-main to use the qtx API.
Still we might have a default script to load to facilitate basic JS functionalities for simple plugins and themes.

Front config

Since qTranslate has no JS script running on the front-end, nothing is done. Custom scripts should also be handled by third-party components.