Merge #17509: gui: save and load PSBT

764bfe4cba35c24f7627cc425d9e7eba56e98964 [psbt] add file size limit (Sjors Provoost)
1cd8dc2556b847e11a238b9e69493cd8fbeecc6c [gui] load PSBT (Sjors Provoost)
f6895301f768220f3ea70231d5cc5b45ecbf4488 [gui] save PSBT to file (Sjors Provoost)
1d05a9d80b1211b47af465ba6958b0ec5a8c33ab Move DEFAULT_MAX_RAW_TX_FEE_RATE to node/transaction.h (Sjors Provoost)
86e22d23bb90383971a68ead0666f225ddd632fb [util] GetFileSize (Sjors Provoost)
6ab3aad9a51cc5e97a8e2ae7dbd5082272163c30 [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  https://github.com/bitcoin/bitcoin/pull/17509/commits/764bfe4cba35c24f7627cc425d9e7eba56e98964
  achow101:
    ACK 764bfe4cba35c24f7627cc425d9e7eba56e98964
  jb55:
    Tested ACK 764bfe4cba35c24f7627cc425d9e7eba56e98964
  jonatack:
    ACK 764bfe4c
  promag:
    Code review ACK 764bfe4cba35c24f7627cc425d9e7eba56e98964.

Tree-SHA512: d284ed6895f3a271fb8ff879aac388ad217ddc13f72074725608e1c3d6d90650f6dc9e9e254479544dd71fc111516b02c8ff92158153208dc40fb2726b37d063

1 files changed, 6 insertions, 0 deletions

diff --git a/src/util/system.cpp b/src/util/system.cpp
index 69a7be96dc..0790ea0d48 100644
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -141,6 +141,12 @@ bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes)
     return free_bytes_available >= min_disk_space + additional_bytes;
 }
 
+std::streampos GetFileSize(const char* path, std::streamsize max) {
+    std::ifstream file(path, std::ios::binary);
+    file.ignore(max);
+    return file.gcount();
+}
+
 /**
  * Interpret a string argument as a boolean.
  *