Proper rewrite for 4.1.1 and upwards

[Neroware]

While developing NativeGodotRx as a GDExtension, I learned a lot of techniques to improve this project. The changes I would want to make in a future refactors:

• Disposables dispose automatically when destroyed: This is a tough one. When I developed this extension, I was unable to properly pull this off. Using the 'self' reference during NOTIFICATION_PREDELETE always results in a Null Reference. I found a trick now. Assigning a member reference and then using unreference() gives us a behavior like std::enable_shared_from_this<>. This way, it is actually possible to keep the instance usable in GDScript
• Proper thread management: See corresponding issue.
• WeakKeyDictionary: See corresponding issue. I see this as a bug.
• Lock-Guards: I cannot see, why this is not a standard thing in GDScript. My god, it is awful to call lock() and unlock() every time we want to ensure thread-safety. Using the trick described earlier, I can now do them myself simplifying the code drastically.

[Neroware]

Great news: I made a proper rewrite of GodotRx for Godot 4.1 fixing a lot of issues from the previous 4.0 version:

• Disposables dispose when out of scope.
• Locks now handled via LockGuards
• WeakKeyDictionary fixed (increases subscription performance drastically)
• ThreadManager introduced cleaning up all finished threads passively
• Fixed memory leaks of ReactiveProperties/ReactiveCollections
• Observable constructors now work with iterables (generators always possible now)
• Fully committed to float as type for time units
• Fixed inheritance madness of Subject, PeriodicScheduler and GodotSignalScheduler through redundancy (ugly but a lack of multi-inheritance has forced my hand)