Add preferred-editor dropdown to manager and editor screens

[Saereth]

New SFMPreferredEditorDropdown widget listing all registered text editors

• Selector in ManagerScreen, V1, V2, and Draw; in-editor switching carries the current content into the new editor
• New SFMPreferredEditorChangedEvent posted on the game bus for addons
• Route showExampleListScreen/showChangelog through the preferred editor
• Save config values after set() (since apparently 26.1 no longer persists on set)
• Removed the IDE-flagged V1/V2 picker from the editor config screen