Add actions-on-double-click option (experimental)

[clansty]

Description

Add --actions-on-double-click as a new configurable action list, consistent with the existing actions-on-enter, actions-on-escape, and actions-on-right-click options.

When actions-on-double-click is not set (empty), double-click does nothing.

Marked as experimental since the future implementation of #89 (generic trigger-action bindings) may provide a more general mechanism for this.

Use case

Double-click to confirm/save is a common gesture in screenshot annotation workflows. Both Snipaste and PixPin use double-click to confirm/save in their annotation modes.

Implementation notes

• Detected via n_pressed >= 2 on primary button click.
• Routed through process_actions (from Add Undo and UndoOrExit action variants for action chains #485), so all action variants including undo and undo-or-exit work.
• No fallback behavior when unconfigured — simply does nothing.

Depends on: #485
Related discussion: #481