Make loadingOverlay public

[healermagnus]

Is your feature request related to a problem? Please describe.
We are subclassing the existing library quite successfully, but I keep tripping on the LoadingOverlay, which takes the very useful name loadingOverlay.

Unfortunately this member variable is marked private, so when subclassing SPSApplication I am stuck with this variable that I don't want, can't subclass, can't access from a subclass, etc., and also I can't make a variable with that useful name.

Describe the solution you'd like
For the other class members from Epic, like afkOverlay and connectOverlay, etc, I have a public member I can override and subclass and it's awesome.

I would like for line 13 of SPSApplication.ts to be:

public loadingOverlay TextOverlay;

That would solve what is an otherwise awesome library to use, and thank you for it.

Describe alternatives you've considered
The alternative solution I've been relegated to is basically ignoring this class member, and subclassing around anything that refers to it.

Additional context
I was trying to submit a PR with this change, but I don't have that level of access to the repository.