Widget: Make contextless widget construction work

Due to the fact the widget factory code is now in strict mode, the check for
being called without using the `new` keyword started breaking if you save the
widget constructor to a variable before calling it:
```js
var customWidget = $.custom.customWidget;
customWidget( {}, elem );
```
as then `this` is undefined and checking for `this._createWidget` crashes.
Account for that with an additional check.

Fixes gh-2015
Closes gh-2019

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