[Qt, OSX] QProgressBar CPU-Issue workaround

fixes #5295
Remove unused ecwrapper code
[Qt, OSX] move QProgressBarMac to guiutil.h
Merge pull request #5296

0ceab00 [Qt, OSX] move QProgressBarMac to guiutil.h (Jonas Schnelli)
6093aa1 [Qt, OSX] QProgressBar CPU-Issue workaround (Jonas Schnelli)
Merge pull request #5313

230f7a8 Remove unused ecwrapper code (Pieter Wuille)
gmp needed to build on OSX
[Wallet] Add global boolean whether to send free transactions (default=true)
[Wallet] Add global boolean whether to pay at least the custom fee (default=true)
[Wallet] Prevent user from paying a non-sense fee
[Qt] Add Smartfee to GUI
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)
doc: add libgmp dependency for secp256k1

Also reorganize the document a bit.