kanji browser: add 'copy to clipboard' button

When looking for kanji in "search for kanji" window, it's possible to easily copy resulting kanji string with single button click.

It's also possible to copy kanji and it's meanings from main window.

But in kanji browser mode it's not intuitively obvious that user may copy kanji by selecting big image of kanji with line numbers printed on it and copying.

This change makes copying kanji from kanji browser more intuitive by providing a button for copying last viewed kanji.