LYN-2723: Fixes issues with bad project or engine paths (#369)

* Setup NativeUIRequests as an AZ::Interface.  Adds a NativeUISystemComponent to AzFramework Application.

* Renames NativeUISystemComponent (class) to NativeUISystem, since it's no longer a Component.

* Minor update to SettingsRegistryInterface::Remove doc-comments for accuracy.

* Fixes to make an early fatal shutdown of Editor occur without crash.

* LYN-2723: Updates startup to handle errors: engine root is empty, no valid project.json found (mismatched engine name), or bad project path (launch project picker dialog).

* LYN-2723: Minor formatting/spelling edits.

* LYN-2723: Moves ParseCommandLine from ComponentApplication to SettingsRegistryMergeUtils so it can be used in more places.

* Misc fixes. 'wait_for_connect' setting wasn't being properly applied to AP connection settings. Fix infinite loop in CCmdLine::Next.

* LYN-2723: Addresses review feedback.

* LYN-2723: Reverts some changes that caused a unit test to fail.

* LYN-2723: Reverts one more change that was unnecessary.

Code/Sandbox/Editor/main.cpp

@@ -30,17 +30,14 @@ int main(int argc, char* argv[])
     [[maybe_unused]] const bool loaded = handle->Load(true);
     AZ_Assert(loaded, "EditorLib could not be loaded");
 
+    int ret = 1;
     if (auto fn = handle->GetFunction<CryEditMain>(CryEditMainName); fn != nullptr)
     {
-        const int ret = AZStd::invoke(fn, argc, argv);
-
-        AZ::AllocatorInstance<AZ::OSAllocator>::Destroy();
-        AZ::Environment::Detach();
-
-        return ret;
+        ret = AZStd::invoke(fn, argc, argv);
     }
 
+    handle = {};
     AZ::AllocatorInstance<AZ::OSAllocator>::Destroy();
     AZ::Environment::Detach();
-    return 1;
+    return ret;
 }