fix "Open Command Prompt Here" on windows with qt 5.8

qt 5.8 made QProcess::startDetached()'s behavior consistent with
start(), which means it won't open a console any more. however, qtc
relied on this (unspecified) behavior.

the correct solution is to add a static function startTerminalEmulator()
to ConsoleProcess, which already has most of the necessary code anyway.

Task-number: QTCREATORBUG-17439
Change-Id: Icf04666869ce6593555302a54c49331a29846a99
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Robert Loehning <robert.loehning@qt.io>

src/libs/utils/consoleprocess_unix.cpp

@@ -395,4 +395,10 @@ QStringList ConsoleProcess::availableTerminalEmulators()
     return result;
 }
 
+bool ConsoleProcess::startTerminalEmulator(QSettings *settings, const QString &workingDir)
+{
+    const QString emu = QtcProcess::splitArgs(terminalEmulator(settings)).takeFirst();
+    return QProcess::startDetached(emu, QStringList(), workingDir);
+}
+
 } // namespace Utils