Rebrand

- Change all BlankCoin, Blankcoin, blankcoin, and BLANKCOIN references
to CashCow, Cashcow, cash cow, and CASHCOW

- Change BLNK to COW

- Change Qt wallet icons/images
Change Staking Reward

Proof of Stake: Definite
"Staking" Rebranding

Stake/Staking replaced with Steak/Milk/Milking depending on context.

Rule of thumb:
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- the verb “stake” replaced with “milk”
- the noun “stake” replaced with “steak”