Add fixed-mouse-position wheel zoom action (Google Maps style)

[waka4g]

First of all, thank you for creating and maintaining this wonderful library! I'm using JKQtPlotter for an internal tool at my company, and it has been incredibly helpful.

I wanted to add a new zoom mode for mouse wheel operation, so I'm submitting this PR. I would appreciate it if you could consider this contribution.

This adds new mouse wheel zoom actions that keep the position under the mouse cursor fixed during zooming (Google Maps style zoom behavior).

Summary

Add new mouse wheel zoom actions that keep the position under the mouse cursor fixed during zooming (Google Maps
style zoom behavior).

New Features

• jkqtpmwaZoomFixedMouseByWheel - Zoom with fixed mouse position
• jkqtpmwaZoomFixedMouseByWheelAndTrackpadPan - Same as above with trackpad pan gesture support

Changes

• Added new enum values to JKQTPMouseWheelActions
• Added string conversion functions for the new actions
• Implemented fixed-mouse-position zoom logic in wheelEvent()
• Works for both plot area and axis-only zooming
• Updated user_interaction example to demonstrate the new options

Usage

plotter->registerMouseWheelAction(Qt::NoModifier,
    JKQTPMouseWheelActions::jkqtpmwaZoomFixedMouseByWheel);

Behavior Comparison

Action	Behavior
jkqtpmwaZoomByWheel	Mouse position becomes the center of the new view
jkqtpmwaZoomFixedMouseByWheel	Mouse position stays fixed on screen (Google Maps style)

[jkriege2]

Thanks for the contribution!