Polish the Qt plugin wizard

- Add overrides for pure virtual base class functions
- Modernize C++ in the generated code
- Do not build into the Qt installation directory by default
- Rename misleading "Qt4Plugin" enum
- Remove Qt 4 compatibility code
- Remove some excess newlines

Fixes: QTCREATORBUG-21207
Change-Id: I50641eddf9084ef3eb8199684aec639c5e5d71fc
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/qmakeprojectmanager/wizards/qtprojectparameters.cpp

@@ -100,14 +100,14 @@ void QtProjectParameters::writeProFile(QTextStream &str) const
     case SharedLibrary:
         str << "TEMPLATE = lib\n\nDEFINES += " << libraryMacro(fileName) << '\n';
         break;
-    case Qt4Plugin:
+    case QtPlugin:
         str << "TEMPLATE = lib\nCONFIG += plugin\n";
         break;
     default:
         break;
     }
 
-    if (!targetDirectory.isEmpty())
+    if (!targetDirectory.isEmpty() && !targetDirectory.contains("QT_INSTALL_"))
         str << "\nDESTDIR = " << targetDirectory << '\n';
 
     if (qtVersionSupport != SupportQt4Only) {