Sort out instance and static methods

[zenflow]

Right now, for backwards compatibility, all instance methods are also exposed as static methods, via proxy methods defined here:

sweetalert2/src/SweetAlert.js

Lines 59 to 66 in 5efb897

	// Proxy to instance methods to constructor, for now, for backwards compatibility
	Object.keys(instanceMethods).forEach(key => {
	SweetAlert[key] = function (...args) {
	if (currentInstance) {
	return currentInstance[key](...args)
	}
	}
	})

We should

• drop this block of code
• change the appropriate static methods to instance methods (e.g. Swal.getTitle() -> swal.getTitle()). Should make sure that calling these methods apply to that particular instance, and not the currently active swal, since they can be different. (Note: there are no instance methods that should become static methods)
• add static method like Swal.getCurrentInstance(), which will be generally useful, and also allow for easy migration in situations where a user is using one of the methods that were changed to an instance method, but they didn't save a reference to the instance
• Split documentation on methods into static methods and instance methods

[limonte]

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