Implements IHasAccentColour for conformity to rest of project. Also fixes a nullref when opening the login menu.
# Conflicts: # osu.Game/Graphics/UserInterface/OsuPasswordTextBox.cs
Some people, myself included, were annoyed by the rotation starting too soon (especially when dragging up and left one pixel).
Fixes them not working at all, too.
Uses GetContainingInputManager instead, as per framework changes.
We need to expose EdgeEffect as it's used in places.