Moved material editing from component window to asset editor

- EditorContentManager now returns null if asset wasn't found (instead of crashing)
- FixUp asset identifier when dragging from asset browser
- Lock current asset in properties editor

GlitchyEditor/src/EditWindows/ContentBrowserWindow.bf

@@ -197,6 +197,8 @@ namespace GlitchyEditor.EditWindows
 				if (fullpath.StartsWith(_manager.ContentDirectory, .OrdinalIgnoreCase))
 					fullpath.Remove(0, _manager.ContentDirectory.Length);
 
+				Path.Fixup(fullpath);
+
 				ImGui.SetDragDropPayload("CONTENT_BROWSER_ITEM", fullpath.CStr(), (.)fullpath.Length, .Once);
 
 				ImGui.EndDragDropSource();