Update some files with Qt 5.15.2 best practices in mind

There was some discussion about updating to Qt6 and I figured I would
work on some smaller parts. For Windows platform the WinMain function has moved
from the Qt5::WinMain to a new one called Qt6::EntryPointPrivate

Also Qt5 supports versionless CMake targets
https://www.qt.io/blog/versionless-cmake-targets-qt-5.15

These other changes in this commit are to support Qt6, but in ways that don't mess with Qt5.

src/yuzu/bootmanager.cpp: Qt6 complains about not being able to know to use QPoint or QPointF, picking QPoint
src/yuzu/bootmanager.h: Qt6 prefers that QStringList.h be included rather than an empty class definition
src/yuzu/configuration/configure_system.cpp: toULongLong intends to return unsigned 64 bit integer, but
   Settings::values.rng_seed is only 32 bits wide
src/yuzu/game_list.cpp: Qt6 returns a different datatype for QStringList.length than Qt5,
   it used to be int, but in Qt6 its now qsizetype
src/yuzu/loading_screen.cpp: Qt5's for QStyleOption.init say to switch to initFrom.
   The QStyleOption.init doesn't exist in Qt6
src/yuzu/main.cpp: Another QPointer and QStringList.size, lets standardize on size()

src/yuzu/loading_screen.h

@@ -7,6 +7,7 @@
 #include <memory>
 #include <QString>
 #include <QWidget>
+#include <QtGlobal>
 
 #if !QT_CONFIG(movie)
 #define YUZU_QT_MOVIE_MISSING 1
@@ -88,4 +89,6 @@ private:
     std::size_t slow_shader_first_value = 0;
 };
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 Q_DECLARE_METATYPE(VideoCore::LoadCallbackStage);
+#endif