strings.xml & strings_help.xml - Korean language translation: Replace every "bitcoin" to "dogecoin"

[HotoRas]

Every commit enclosed to this pull request is written in pure github:

There's a pull request about this already exist at #113 but there exist some unmodified strings on strings_help.xml. This time, I manually searched them one-by-one and modified. This pull requestion also has a translation of non-translated-or-wrongly-translated extended public key statement.

Changes

• Modified every 비트코인(means bitcoin) to 도지코인(means dogecoin)
• Translated extended public key to 확장 공개키, and replaced some 확장 개인키(means extended private key) to them because the key data defines about extended public key

Notes

The reason why I made this pull request is: I found NO dogecoin is stated other than the main logo and live price chart, but bitcoin is stated as the replacement.