Utils: Introduce QtcProcess::toStandaloneCommandLine()

The goal is to make a QtcProcess incl. environment, working directory,
executable and arguments testable, e.g. in a terminal. The new
toStandaloneCommandLine() returns a string containing the command line
for a call of "env" the data of the QtcProcess instance as parameters.

To be used like:
  qDebug().noquote() << qtcProc.toStandaloneCommandLine();

..and the debug output can be pasted into a terminal.

Change-Id: Ib6cbea290e1eff3279d6e0a67076a624312af879
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/qtcprocess.h

@@ -193,6 +193,8 @@ public:
 
     void setStandardInputFile(const QString &inputFile);
 
+    QString toStandaloneCommandLine() const;
+
 signals:
     void started();
     void finished();