Can't deselect an item in the selection group menu

[benjaminforras]

Describe the bug
Can't deselect a selected item in the selection group menu.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://trimox.github.io/angular-mdc-web/#/angular-mdc-web/menu-demo/examples
2. Click on 'Show Selection Group Menu'
3. Select anything.
4. Now try to deselect.

Expected behavior
To select an item and to deselect it.
Also it would be a nice touch if I don't have to reopen the menu every time I check something.

What Angular MDC version are you using?
v3.2.0

What browser(s) is this bug affecting?:
I only tried with the latest Google Chrome.

[trimox]

@TryHardDood Thanks for reporting this.

Problems seems to be in getSelectedSiblingOfItemAtIndex(index: number); always returning -1.
https://github.com/trimox/angular-mdc-web/blob/master/packages/menu/menu.ts#L108

Let ya know..

[trimox]

Got it. PR incoming.

Also it would be a nice touch if I don't have to reopen the menu every time I check something.

I'll have to submit separate PR for surface closing.