Templates: Use C++17 compact namespaces in the plugin template

Change-Id: If4c8540e4d1d9b42efcd1c039c28f85bd0fb98fd
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

share/qtcreator/templates/wizards/qtcreatorplugin/myplugin.cpp

@@ -13,8 +13,7 @@
 #include <QMainWindow>
 #include <QMenu>
 
-namespace %{PluginName} {
-namespace Internal {
+namespace %{PluginName}::Internal {
 
 %{CN}::%{CN}()
 {
@@ -75,5 +74,4 @@ void %{CN}::triggerAction()
                              tr("This is an action from %{PluginName}."));
 }
 
-} // namespace Internal
-} // namespace %{PluginName}
+} // namespace %{PluginName}::Internal

share/qtcreator/templates/wizards/qtcreatorplugin/myplugin.h

@@ -9,8 +9,7 @@
 
 #include <extensionsystem/iplugin.h>
 
-namespace %{PluginName} {
-namespace Internal {
+namespace %{PluginName}::Internal {
 
 class %{CN} : public ExtensionSystem::IPlugin
 {
@@ -29,8 +28,7 @@ private:
     void triggerAction();
 };
 
-} // namespace Internal
-} // namespace %{PluginName}
+} // namespace %{PluginName}::Internal
 
 @if ! '%{Cpp:PragmaOnce}'
 #endif // %{GUARD}