Add undo/redo support to settings menu

[rekcilyssup]

Closes #1615.

Undo and redo (Ctrl+Z / Ctrl+Y) previously only worked in the shortcuts section of the settings menu. This change routes all setting changes through the menu's undo_redo so they can be undone/redone.

Changes:

• UndoRedoRef now passes merge_mode/backward_undo_ops through to create_action(), and exposes is_committing_action().
• settings_menu.gd passes its undo_redo to every SettingsContentGeneric instance.
• settings_content_generic.gd wraps the setting setter in an undo/redo action:
  • MERGE_ENDS is used so continuous edits (e.g. dragging a color) merge into a single undo step.
  • Actions are only created when the value actually changes, since widget initialization during content rebuilds otherwise creates no-op actions that pollute the history.
  • The content rebuilds after undo/redo (but not during commits, checked via is_committing_action()) so widgets reflect reverted values.
  • The version_changed connection is cleaned up on tree_exiting.

Verified with Godot 4.6 stable (headless): checkbox/dropdown/numeric settings all set -> undo -> redo correctly, merged color edits undo in one step, and no stale UI state after reverting.

[MewPurPur]

Awesome! I set up the architecture a few weeks ago but didn't know if I'll get to this anytime soon. I'll test and review this posthaste.

[MewPurPur]

Thanks!