Add a checkbox option: highlight changed characters

[BSBMteam]

If a “Highlight changed characters" checkbox is checked: Highlight the affected characters in the results box.
也可以使用這種翻譯「標記已轉換的字」

For example,
存储空间即将用完

Add a color menu to choose highlight color (for color-blind people)

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Write a message if no characters were changed. For example:

No characters have been converted! All characters in both character sets happen to be the same.
原文沒轉換任何字！所用文本框中的所有字正好一樣。

[CXuesong]

Good idea!

有空了我去搞一个。不过如此一来就不能使用组件库内置的多行文本框了（逃