Event handlers overwrite old handlers

[1vanK]

This cause problems in web builds

• https://urho3d.io/samples/36_Urho2DTileMap.html
• https://urho3d.io/samples/32_Physics2DConstraints.html

Already subscribed event handler for E_MOUSEBUTTONDOWN is overwritten in function InitMouseMode(). Perhaps we need to allow multiple event handlers as in C#

[github-actions]

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