fix: Change loadButton tag to div (fix #137)

[junghwan-park]

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Description

Change UI Load button Tag 'button' to 'div'.
Because of input tag in button tag is invalid html, the button in Firefox does not working as expected.

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[arifhussain353]

@junghwan-park Thanks for this fix. I think it should also be changed in controls.js tui-image-editor-controls-buttons for footer view buttons? https://github.com/nhnent/tui.image-editor/blob/1562a4434f50241c84928ec539e023eee23ecb2d/src/js/ui/template/controls.js#L51

[dongsik-yoo]

수고하셨습니다. 리뷰 완료했습니다.

[jinwoo-kim-nhn]

수고하셨습니다.