dynamically resize spalsh screen image based on screen resolution

This should resolve some of the Huge splash screens we get with the workbench sometimes.

It also means we only have to maintain one splash screen image and not two resolutions of the same thing.

qt/applications/workbench/workbench/app/mainwindow.py

@@ -121,17 +121,19 @@ MAIN_APP = qapplication()
 atexit.register(qCleanupResources)
 
 
-def _get_splash_image_name():
+def _get_splash_image():
     # gets the width of the screen where the main window was initialised
     width = QGuiApplication.primaryScreen().size().width()
+    height = QGuiApplication.primaryScreen().size().height()
 
-    if width > 2048:
-        return ':/images/MantidSplashScreen_4k.jpg'
-    else:
-        return ':/images/MantidSplashScreen.png'
-
+    # the proportion of the whole window size for the splash screen
+    splash_screen_scaling = 0.2
+    return QPixmap(':/images/MantidSplashScreen_4k.jpg').scaled(width * splash_screen_scaling,
+                                                                height * splash_screen_scaling,
+                                                                Qt.KeepAspectRatio,
+                                                                Qt.SmoothTransformation)
 
-SPLASH = QSplashScreen(QPixmap(_get_splash_image_name()),
+SPLASH = QSplashScreen(_get_splash_image(),
                        Qt.WindowStaysOnTopHint)
 SPLASH.show()
 SPLASH.showMessage("Starting...", Qt.AlignBottom | Qt.AlignLeft