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/Framework/AzToolsFramework/AzToolsFramework/Application/ToolsApplication.cpp

@@ -284,6 +284,10 @@ namespace AzToolsFramework
     void ToolsApplication::Start(const Descriptor& descriptor, const StartupParameters& startupParameters/* = StartupParameters()*/)
     {
         Application::Start(descriptor, startupParameters);
+        if (!m_isStarted)
+        {
+            return;
+        }
 
         m_editorEntityManager.Start();