Merge #11316: [qt] Add use available balance in send coins dialog (CryptAxe, promag)

Merge #11316: [qt] Add use available balance in send coins dialog (CryptAxe, promag)

d052e3847 [qt] Add use available balance in send coins dialog (CryptAxe)

Pull request description:

  This is an alternative to #11098 to handle #11033 where a new button `Use available balance` is added to each entry. When activated, the available balance is calculated by using the coin control (if any) and then it's subtracted the remaining recipient amounts. If this amount is positive then the `Subtract fee from amount` is automatically selected.

  Comparing to #11098, this has the advantage to avoid the fair amount division over the recipients and allows to fine adjust the amounts in multiple iterations.

  Started from @CryptAxe commit 89e9eda to credit some code.

  <img width="965" alt="screen shot 2017-09-13 at 01 32 44" src="https://user-images.githubusercontent.com/3534524/30354518-e1bee31c-9824-11e7-9354-300aa63cdfd0.png">
  <img width="964" alt="screen shot 2017-09-13 at 01 44 57" src="https://user-images.githubusercontent.com/3534524/30354598-5731ac9c-9825-11e7-9d5f-8781988ed219.png">

Tree-SHA512: 01d20c13fd8b6c2a0ca1d74d3a9027c6922e6dccd3b08e59d5a72636be7072ed5eca7ebc5d431299497dd3374e83753220ad4174d8bc46dadb4b2f54973036a5