compositor: Solid color for splash screen

Splash screen is white until the background component is loaded asynchronously
and takes more time to load compared to the desktop itself, resulting in a
white rectangle that is faded into the desktop.

Use the same solid color from the Plymouth and SDDM themes instead, this will
also make the transition feel smooth.

Closes: #74

compositor/qml/screens/SplashScreen.qml

@@ -25,12 +25,13 @@
  ***************************************************************************/
 
 import QtQuick 2.0
-import QtGraphicalEffects 1.0
 import QtQuick.Controls 2.0
 import QtQuick.Controls.Material 2.0
-import Liri.Shell 1.0
 
-Background {
+Rectangle {
+    // Same color as Plymouth and SDDM
+    color: Material.color(Material.BlueGrey, Material.Shade800)
+
     Material.theme: Material.Dark
     Material.primary: Material.Blue
     Material.accent: Material.Blue