Add support for nested global entries

[revolter]

Please review each commit separately, as this PR is logically split into multiple "slow" transitions from the current implementation to the new one.

[revolter]

In Swift, if you don't use the closure argument, like this:

FLEXManager.shared.registerGlobalEntry(withName: "Name") { _ in
    print("Log")
}

you now get the build error:

Ambiguous use of 'registerGlobalEntry'

and the only workaround that I can think of is changing the call to:

FLEXManager.shared.registerGlobalEntry(withName: "Name") { (_: UITableViewController) in
    print("Log")
}

or

FLEXManager.shared.registerGlobalEntry(withName: "Name") { (_: FLEXUserGlobalEntriesContainer) in
    print("Log")
}

But this means that you have to look into FLEX's files to find this out, if you're new to it.

---

So, I will rename the new method (from Swift's POV, the first parameter name), to avoid this ambiguity:

FLEXManager.shared.registerNestedGlobalEntry(withName: "Name") { _ in
    print("Log")
}

[revolter]

@NSExceptional, do you have time to look over this? 😁

[NSExceptional]

Not at the moment no; I was actually in a serious car accident on Tuesday and just got out of the hospital yesterday 🤕

I reviewed a PR yesterday but it was just a small bug fix

[NSExceptional]

I'll try to get to this when I can use my hand again

[revolter]

OMG, I'm so sorry to hear that 😱 Forget about this PR, get well soon 🤗

[revolter]

I also added support for optionally customizing the cell accessory type:

Before	After