Implement multi-session support and welcome screen in EditorManager

- Removed the homepage display logic and replaced it with a welcome screen that appears when no ROM is loaded or no active editors are present.
- Enhanced session management by iterating through all sessions to check for active editors, allowing for better multi-session docking.
- Introduced new methods for generating unique editor titles based on session context and added feature flags for per-session configurations.
- Added safeguards for ROM loading state checks in editor methods to prevent operations on unloaded ROMs.

src/app/editor/dungeon/dungeon_editor.cc

@@ -46,6 +46,10 @@ void DungeonEditor::Initialize() {
 }
 
 absl::Status DungeonEditor::Load() {
+  if (!rom_ || !rom_->is_loaded()) {
+    return absl::FailedPreconditionError("ROM not loaded");
+  }
+  
   auto dungeon_man_pal_group = rom()->palette_group().dungeon_main;
 
   // Use room loader component for loading rooms