Merge pull request #5200

c1c9d5b [Qt] Add Smartfee to GUI (Cozz Lovan)
e7876b2 [Wallet] Prevent user from paying a non-sense fee (Cozz Lovan)
ed3e5e4 [Wallet] Add global boolean whether to pay at least the custom fee (default=true) (Cozz Lovan)
0ed9675 [Wallet] Add global boolean whether to send free transactions (default=true) (Cozz Lovan)