[kstyle] Use KWindowSystem to determine on which platform we are

Less code duplication and works better in case of e.g. flatpak.

kstyle/breezehelper.cpp

@@ -1420,7 +1420,7 @@ namespace Breeze
     {
         #if BREEZE_HAVE_X11
         #if QT_VERSION >= 0x050000
-        static const bool s_isX11 = QX11Info::isPlatformX11();
+        static const bool s_isX11 = KWindowSystem::isPlatformX11();
         return s_isX11;
         #else
         return true;
@@ -1435,7 +1435,7 @@ namespace Breeze
     bool Helper::isWayland()
     {
         #if QT_VERSION >= 0x050000
-        static const bool s_isWayland = qApp->platformName().startsWith(QLatin1String("wayland"));
+        static const bool s_isWayland = KWindowSystem::isPlatformWayland();
         return s_isWayland;
         #else
         return false;