add move selection

[nellyvo]

Fixes #34

Pull Request Summary

This pull request introduces an enhancement to the selection tool that allows users to move the selection (and its underlying drawing) without copying it when no modifier key is pressed.

Key Changes:

• New Functionality: When the selection is locked and the user clicks inside it without holding the Alt key, the content is moved from its original location (which is cleared to transparent) to the new position.
• Preserved Behavior: The existing copy-and-move behavior using the Alt key remains unchanged.
• Code Modifications: Only the eventhandlers.js related to selection manipulation have been updated.
• Testing: The new feature has been thoroughly tested and works as expected.

[Kully]

@nellyvo Nice, this is really good work. It works as I pictured it.

One thing I noticed is that once you make a selection and then click and do not move the cursor (mousedown):

• the drawing within the bounds of the selection is not visible
• the cursor is still the crosshair and not the dragging icon

Here's a video to show you:
https://github.com/user-attachments/assets/eb0c1df7-f819-49f0-9d1f-2de251a2e99c

Can you make sure that the drawing is visible and the cursor looks like it does during drag mode?