Add set_visibility and show_zoom_buttons

[ninerforce]

feat: Add functionality to toggle visibility of zoom buttons

Response to Issue #147 https://github.com/TomSchimansky/TkinterMapView/issues/147

• Introduced set_visibility method in CanvasButton to dynamically show or hide buttons by controlling their canvas item states.
• Added toggle_zoom_buttons method in TkinterMapView to manage the visibility of zoom buttons (+ and -) using the new set_visibility feature.
• Ensures clean UI manipulation by hiding buttons when not needed, while preserving functionality for future re-enabling.

This update enhances the flexibility of the map widget interface.