Make screens always load async (fixes stutter when entering editor)

[peppy]

Makes editor not stutter on load, amongst other screens.

• Depends on ppy/osu-framework#1490.
• Mege #2438 first to fix the error that CI will begin to report with this branch.

[smoogipoo]

Attempting to enter song select too fast breaks.

[smoogipoo]

Starting player with dummy beatmap selected:

System.InvalidOperationException: Cannot push an already exited screen.
  at osu.Framework.Screens.Screen.Push (osu.Framework.Screens.Screen screen) [0x0006b] in <e430228582a241528c881254853c30bc>:0 
  at osu.Game.Screens.Play.PlayerLoader.<pushWhenLoaded>b__19_1 () [0x0001e] in <98f8fafee31b4b1b871b9d519aeed96d>:0 
  at osu.Framework.Threading.ScheduledDelegate.RunTask () [0x0000e] in <e430228582a241528c881254853c30bc>:0 
  at osu.Framework.Threading.Scheduler.Update () [0x0024b] in <e430228582a241528c881254853c30bc>:0 
  at osu.Framework.Graphics.Drawable.UpdateSubTree () [0x000a1] in <e430228582a241528c881254853c30bc>:0 

fixed