Use static_cast instead of C-style casts for non-fundamental types

A C-style cast is equivalent to try casting in the following order:

1. const_cast(...)
2. static_cast(...)
3. const_cast(static_cast(...))
4. reinterpret_cast(...)
5. const_cast(reinterpret_cast(...))

By using static_cast<T>(...) explicitly we avoid the possibility
of an unintentional and dangerous reinterpret_cast. Furthermore
static_cast<T>(...) allows for easier grepping of casts.

src/qt/splashscreen.cpp

@@ -38,7 +38,7 @@ SplashScreen::SplashScreen(Qt::WindowFlags f, const NetworkStyle *networkStyle)
     float fontFactor            = 1.0;
     float devicePixelRatio      = 1.0;
 #if QT_VERSION > 0x050100
-    devicePixelRatio = ((QGuiApplication*)QCoreApplication::instance())->devicePixelRatio();
+    devicePixelRatio = static_cast<QGuiApplication*>(QCoreApplication::instance())->devicePixelRatio();
 #endif
 
     // define text to place