Add MutableDisposable and use for CursorBlinkStateManager

[Tyriar]

This fixed a memory leak: #4659

But the registration here isn't the best as if blinking is toggled off and on again there will be 2 values registered, despite only the last one being active. VS Code has a nice pattern for this with MutableDisposable: https://github.com/microsoft/vscode/blob/f0f36dff5efa066df997197e1d3a78e2727b70a9/src/vs/base/common/lifecycle.ts#L320-L376