Migrate ProjectManager to dependency injection

- Created IProjectManager interface
- ProjectManager now implements IProjectManager
- Removed ProjectManager::Get() singleton
- Added IEditorContext::GetProjectManager()
- ProjectPanel now uses m_context->GetProjectManager() instead of singleton
- EditorContext owns ProjectManager instance

editor/src/Core/IEditorContext.h

@@ -8,6 +8,7 @@ namespace Editor {
 
 class EventBus;
 class ISelectionManager;
+class IProjectManager;
 
 class IEditorContext {
 public:
@@ -16,6 +17,7 @@ public:
     virtual EventBus& GetEventBus() = 0;
     virtual ISelectionManager& GetSelectionManager() = 0;
     virtual void* GetSceneManager() = 0;
+    virtual IProjectManager& GetProjectManager() = 0;
     
     virtual void SetProjectPath(const std::string& path) = 0;
     virtual const std::string& GetProjectPath() const = 0;