Set placeholderText from code

Avoids a hard requirement of Qt 5.3; only 5.1 is
needed.

src/GUI/Launcher.ui

@@ -666,9 +666,6 @@
           </property>
           <item>
            <widget class="QPlainTextEdit" name="commandLineArgs">
-            <property name="placeholderText">
-             <string>--option=value --prop:/sim/name=value</string>
-            </property>
            </widget>
           </item>
          </layout>

src/GUI/QtLauncher.cxx

@@ -671,6 +671,10 @@ QtLauncher::QtLauncher() :
     m_ui.reset(new Ui::Launcher);
     m_ui->setupUi(this);
 
+#if QT_VERSION >= 0x050300
+    // don't require Qt 5.3
+    m_ui->commandLineArgs->setPlaceholderText("--option=value --prop:/sim/name=value");
+#endif
     for (int i=0; i<4; ++i) {
         m_ratingFilters[i] = 3;
     }