Utils/ProjectExplorer: Use FilePath for Runnable::workingDirectory

... and in some using code.

Change-Id: I231ea56628908f7d305d13f07eabe8803fe8a791
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -911,7 +911,7 @@ void RunConfigurationSelectionDialog::populate()
                 auto runnable = rc->runnable();
                 const QStringList rcDetails = { runnable.executable.toString(),
                                                 runnable.commandLineArguments,
-                                                runnable.workingDirectory };
+                                                runnable.workingDirectory.toString() };
                 m_rcCombo->addItem(rc->displayName(), rcDetails);
             }
         }