Exit code and stderr/stdout handling for SystemCommandExecutor (#388)

This makes executeCommand return the exit code when finished,
and makes separate signals for standard error and standard out.

This is not backwards compatible because the signal processOutput has
been replaced by processStandardOutput and processStandardError.

systemcommandexecutor.h

@@ -12,45 +12,44 @@ class SystemCommandExecutor : public QObject
 public:
     explicit SystemCommandExecutor(QObject *parent = nullptr) : QObject(parent) {}
 
-    Q_INVOKABLE QString executeCommand(const QString &command) {
+    Q_INVOKABLE int executeCommand(const QString &command) {
         QProcess process;
         QEventLoop loop;
 
         process.setReadChannel(QProcess::StandardOutput);
 
-        auto conn1 = connect(&process, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), &loop, &QEventLoop::quit);
-        auto conn2 = connect(&process, &QProcess::errorOccurred, &loop, &QEventLoop::quit);
-        auto conn3 = connect(&process, &QProcess::readyReadStandardOutput, [&]() {
-            emit processOutput(QString::fromUtf8(process.readAllStandardOutput()));
+        auto quitHandler = connect(&process, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished), &loop, &QEventLoop::quit);
+        auto errHandler = connect(&process, &QProcess::errorOccurred, &loop, &QEventLoop::quit);
+        auto stdOutHandler = connect(&process, &QProcess::readyReadStandardOutput, [&]() {
+            emit processStandardOutput(QString::fromUtf8(process.readAllStandardOutput()));
         });
-        auto conn4 = connect(&process, &QProcess::readyReadStandardError, [&]() {
-            emit processOutput(QString::fromUtf8(process.readAllStandardError()));
+        auto stdErrHandler = connect(&process, &QProcess::readyReadStandardError, [&]() {
+            emit processStandardError(QString::fromUtf8(process.readAllStandardError()));
         });
 
         process.start("sh", QStringList() << "-c" << command);
 
         if (!process.waitForStarted()) {
-            disconnect(conn1);
-            disconnect(conn2);
-            disconnect(conn3);
-            disconnect(conn4);
-            return "Failed to start process";
+            disconnect(quitHandler);
+            disconnect(errHandler);
+            disconnect(stdOutHandler);
+            disconnect(stdErrHandler);
+            return -1;
         }
 
         loop.exec();
 
-        disconnect(conn1);
-        disconnect(conn2);
-        disconnect(conn3);
-        disconnect(conn4);
+        disconnect(quitHandler);
+        disconnect(errHandler);
+        disconnect(stdOutHandler);
+        disconnect(stdErrHandler);
 
-        QString output = process.readAllStandardOutput();
-        QString errorOutput = process.readAllStandardError();
-        return output + errorOutput;
+        return process.exitCode();
     }
 
 signals:
-    void processOutput(QString);
+    void processStandardOutput(QString);
+    void processStandardError(QString);
 
 };