Move incompatibility tooltip logic to local player mod select overlays

This one turned out to be a bit more involved, due to tooltips being
shared and having the potential of being used somewhere where it
shouldn't be, due to the same content type matching.

That's the reason I've defined a protected `TargetContentType`, to be
able to separate "local player mod tooltips" and regular mod tooltips
apart.

Definitely unsure about the solution, but that's as far as I can think
of right now.

osu.Game/Overlays/Mods/ModButton.cs

@@ -314,6 +314,6 @@ namespace osu.Game.Overlays.Mods
 
         public virtual ITooltip GetCustomTooltip() => new ModButtonTooltip();
 
-        public object TooltipContent => SelectedMod ?? Mods.FirstOrDefault();
+        public object TooltipContent => this;
     }
 }