qt: Use fixed-point arithmetic in amount spinbox

Fixes various issues and cleans up code

- Fixes issue #4500: Amount widget +/- has floating point rounding artifacts
- Amount box can now be emptied again, without clearing to 0

Also aligns the amount to the right, as in other places.
qt: Remove unused functions from BitcoinUnits

Remove two functions that are now unused.
ui: Make sure sendcoinsentry signals only connected once

Move signal connections to constructor where possible.
Merge pull request #4556

29eaa31 ui: Make sure sendcoinsentry signals only connected once (Wladimir J. van der Laan)
2a05101 qt: Remove unused functions from BitcoinUnits (Wladimir J. van der Laan)
91cce17 qt: Use fixed-point arithmetic in amount spinbox (Wladimir J. van der Laan)