Merge #17509: gui: save and load PSBT

764bfe4cba [psbt] add file size limit (Sjors Provoost)
1cd8dc2556 [gui] load PSBT (Sjors Provoost)
f6895301f7 [gui] save PSBT to file (Sjors Provoost)
1d05a9d80b Move DEFAULT_MAX_RAW_TX_FEE_RATE to node/transaction.h (Sjors Provoost)
86e22d23bb [util] GetFileSize (Sjors Provoost)
6ab3aad9a5 [gui] send dialog: split on_sendButton_clicked (Sjors Provoost)

Pull request description:

  This adds:
  * a dialog after Create Unsigned, which lets you save a PSBT file in binary format, e.g. to an SD card
  * a "Load PSBT" menu entry lets you pick a PSBT file. We broadcast the transaction if complete

  ## Save flow
  <img width="482" alt="Schermafbeelding 2020-01-04 om 20 39 34" src="https://user-images.githubusercontent.com/10217/71765684-ba60d580-2f32-11ea-8dea-0c4398eb6e15.png">

  <img width="287" alt="Schermafbeelding 2020-01-04 om 20 40 35" src="https://user-images.githubusercontent.com/10217/71765677-a0bf8e00-2f32-11ea-8172-12dfd34a89f3.png">

  <img width="594" alt="Schermafbeelding 2020-01-04 om 20 41 12" src="https://user-images.githubusercontent.com/10217/71765681-aa48f600-2f32-11ea-8e2c-c4f6bf9f5309.png">

  <img width="632" alt="Schermafbeelding 2020-01-04 om 20 41 28" src="https://user-images.githubusercontent.com/10217/71765691-d19fc300-2f32-11ea-97ff-70f5dd59987a.png">

  By default the file name contains the destination address(es) and amount(s).

  We only use the binary format for files, in order to avoid compatibility hell. If we do want to add base64 file format support, we should use a different extension for that (`.psbt64`?).

  ## Load flow

  Select a file:
  <img width="649" alt="Schermafbeelding 2020-01-04 om 21 08 57" src="https://user-images.githubusercontent.com/10217/71766089-2ba28780-2f37-11ea-875d-074794b5707d.png">

  Offer to send if complete:

  <img width="308" alt="Schermafbeelding 2020-01-04 om 21 09 06" src="https://user-images.githubusercontent.com/10217/71766088-2a715a80-2f37-11ea-807d-394c8b840c59.png">

  Tell user if signatures are missing, offer to copy to clipboard:
  <img width="308" alt="Schermafbeelding 2020-01-04 om 21 15 57" src="https://user-images.githubusercontent.com/10217/71766115-702e2300-2f37-11ea-9f62-a6ede499c0fa.png">

  Incomplete for another reason:

  <img width="309" alt="Schermafbeelding 2020-01-04 om 21 07 51" src="https://user-images.githubusercontent.com/10217/71766090-2c3b1e00-2f37-11ea-8a22-6188377b67a1.png">

ACKs for top commit:
  instagibbs:
    re-ACK  764bfe4cba
  achow101:
    ACK 764bfe4cba
  jb55:
    Tested ACK 764bfe4cba
  jonatack:
    ACK 764bfe4c
  promag:
    Code review ACK 764bfe4cba.

Tree-SHA512: d284ed6895f3a271fb8ff879aac388ad217ddc13f72074725608e1c3d6d90650f6dc9e9e254479544dd71fc111516b02c8ff92158153208dc40fb2726b37d063

src/rpc/rawtransaction.cpp

@@ -40,12 +40,6 @@
 
 #include <univalue.h>
 
-/** Maximum fee rate for sendrawtransaction and testmempoolaccept.
- * By default, a transaction with a fee rate higher than this will be rejected
- * by the RPCs. This can be overridden with the maxfeerate argument.
- */
-static const CFeeRate DEFAULT_MAX_RAW_TX_FEE_RATE{COIN / 10};
-
 static void TxToJSON(const CTransaction& tx, const uint256 hashBlock, UniValue& entry)
 {
     // Call into TxToUniv() in bitcoin-common to decode the transaction hex.