Stop support Qt versions before 5.9

There's just no point to testing these outdated versions.
Everything we build is now Qt 5.9 or newer.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.travis.yml

@@ -130,27 +130,6 @@ matrix:
 #          directories:
 #              - 3pp
 
-    - env: SUBSURFACE_PLATFORM="qt55"
-      os: linux
-      language: c++
-      filter_secrets: false
-      services:
-          - docker
-
-#    - env: SUBSURFACE_PLATFORM="qt56"
-#      os: linux
-#      language: c++
-#      filter_secrets: false
-#      services:
-#          - docker
-
-    - env: SUBSURFACE_PLATFORM="qt57"
-      os: linux
-      language: c++
-      filter_secrets: false
-      services:
-          - docker
-
     - env: SUBSURFACE_PLATFORM="qt59"
       os: linux
       language: c++

core/qt-init.cpp

@@ -24,7 +24,7 @@ void init_qt_late()
 
 	QCoreApplication::setOrganizationName("Subsurface");
 	QCoreApplication::setOrganizationDomain("subsurface.hohndel.org");
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 7, 0)) && defined(Q_OS_LINUX)
+#if defined(Q_OS_LINUX)
 	QGuiApplication::setDesktopFileName("subsurface");
 #endif
 	// enable user specific settings (based on command line argument)

core/qthelper.h

@@ -112,22 +112,6 @@ void moveInVector(Vector &v, int rangeBegin, int rangeEnd, int destination)
 	else if (destination < rangeBegin)
 		std::rotate(it + destination, it + rangeBegin, it + rangeEnd);
 }
-
-// Qt overloads some signals(!) which can't therefore be used in connect() calls with
-// pointers-to-member functions where the signatures of signal and slot don't match perfectly.
-// For this case, Qt 5.7 provides the qOverload<> helper.
-// Since we still support Qt 5.5 let's reimplement it here.
-#if QT_VERSION < 0x050700
-template <typename... Args>
-struct QOverload {
-	template <typename Ret, typename Class>
-	static Ret (Class::*of(Ret (Class::*fun)(Args...)))(Args...)
-	{
-		return fun;
-	}
-};
-#endif
-
 #endif
 
 // 3) Functions visible to C and C++

subsurface-desktop-main.cpp

@@ -214,18 +214,8 @@ exit:
 	ctx.makeCurrent(NULL);
 	surface.destroy();
 	if (glError) {
-#if QT_VERSION < QT_VERSION_CHECK(5, 8, 0)
-		qWarning() << QStringLiteral(VALIDATE_GL_PREFIX
-					     "ERROR: %1.\n"
-					     "Cannot automatically fallback to a software renderer!\n"
-					     "Set the environment variable 'QT_QUICK_BACKEND' with the value of 'software'\n"
-					     "before running Subsurface!\n")
-				      .arg(glError);
-		exit(0);
-#else
 		qWarning() << QStringLiteral(VALIDATE_GL_PREFIX "WARNING: %1. Using a software renderer!").arg(glError);
 		QQuickWindow::setSceneGraphBackend(QSGRendererInterface::Software);
-#endif
 	}
 }