qt: Remove the "Pay only required fee..." checkbox #14608

This adds functions for specifing a min/max value for a
BitcoinAmountField. These options only affect user input, so it's still
possible to use setValue to set values outside of the min/max range. The
existing value will not be changed when calling these functions even if
it's out of range. The min/max range will be reinforced when the field
loses focus.
This also adds `SetAllowEmpty` function which specifies if the field is
allowed to be left empty by the user. If set to false the field will be
set to the minimum allowed value if it's empty when focus is lost.

The custom fee input box now has a minimum value equal to the minimum
required fee. Before a value below the minimum fee could be entered
which was confusing since the minimum fee would still be paid even
though a lower amount was entered.