Add isEqual property to the form

[svenank3r]

Currently when no changes are made to the form (in comparison with the intial value of the form), the manualSave$ will not trigger an output update and therefore you cant submit the form.

In this issue I promised to look into adding a property that exposes the isEqual check that is used during the manualSave.
As my solution does not solve the original issue that was reported, I created this new issue to do the pull request on.

[NgrNxk]

Oh yes, please! I would love to submit without changes! On a side-note: how can I trick ngx-sub-form into thinking the initial value is empty but still set a value with this.input$.next(entity)?