Merge bitcoin#16044: qt: fix opening bitcoin.conf via Preferences on macOS

6e6494b qt: fix opening bitcoin.conf via Preferences on macOS; see bitcoin#15409 (shannon1916)

Pull request description:

  Fix bitcoin#15409. The QT wallet fail to open the configuration file on Mac, when these is no default application for `*.conf` files.

  Here is a feasible way to solve this bug. When `QDesktopServices::openUrl` fails to open `file:///path/bitcoin.conf` with its default application, use `QProcess::startDetached` to run `open -t /path/bitcoin.conf` command instead, so as to open the configuration file with system's default text editor.

ACKs for commit 6e6494:
  hebasto:
    re-ACK 6e6494b
  fanquake:
    tACK bitcoin@6e6494b on macOS 10.14.x

Tree-SHA512: 60e898f4cb77cfd7b8adbc8d33fbebf46bac2a801bdcf40cae15e24b78ad56b1f32358b1879b670623d9f8651dea93961d34269358cea18f4e15b089a8ffcfbf

Author: laanwj

src/qt/guiutil.cpp

@@ -60,6 +60,7 @@
 
 #include <objc/objc-runtime.h>
 #include <CoreServices/CoreServices.h>
+#include <QProcess>
 #endif
 
 namespace GUIUtil {
@@ -399,7 +400,15 @@ bool openBitcoinConf()
     configFile.close();
 
     /* Open bitcoin.conf with the associated application */
-    return QDesktopServices::openUrl(QUrl::fromLocalFile(boostPathToQString(pathConfig)));
+    bool res = QDesktopServices::openUrl(QUrl::fromLocalFile(boostPathToQString(pathConfig)));
+#ifdef Q_OS_MAC
+    // Workaround for macOS-specific behavior; see #15409.
+    if (!res) {
+        res = QProcess::startDetached("/usr/bin/open", QStringList{"-t", boostPathToQString(pathConfig)});
+    }
+#endif
+
+    return res;
 }
 
 ToolTipToRichTextFilter::ToolTipToRichTextFilter(int _size_threshold, QObject *parent) :