Ability to use text input while menu is open

[MurhafSousli]

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

Ability to use the text input while the menu is open, it should work like <auto-complete> but without filtering results.

What is the current behavior?

When <mat-menu> is used on <mat-form-field> and the menu opens it takes the focus from the input

What are the steps to reproduce?

Check this stackblitz

What is the use-case or motivation for changing an existing behavior?

Improves user experience, adds more control over the menu and its behavior

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular: 6.1.0
Material: 6.4.0

Is there anything else we should know?

#9857 is some how related but not the same

[josephperrott]

Closing as duplicate of #9857

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