Support MouseOver For Menu Instead Of Click

#### Feature Description
I would like an option to be available for MdMenu that will open it when the mouse is over the menu element, and close it when the mouse leaves the menu element, or the menu dropdown.

**Provide a brief summary of the feature you would like to see.**

**This is duplicate of the Issue #2080**. Our team encountered the same problem in our development and we had to use the hacky method @Marshal27 provided in https://github.com/angular/components/issues/2080#issuecomment-444488219.

It looks like alot of people in this thread legitimately wanted this feature to use, and there was even talks about how it would work, then just died out for some reason, with some later outrage from people about the ignorance of the issue.

I would like to reopen this Issue and once again discuss ways we can integrate the mouseover functionality into the material menu. I can certainly use it and it would benefit me and looks like alot of other people as well.

#### Use Case
**Describe the use case(s) that the proposed feature would enable.**

Use case would be in menu structures where a mouse over functionality would be favorable over button click. For us we are doing a notification icon that would display the type and number of notification on hover.

Non è indicato alcun file di implementazione né alcun test. Inizia esaminando la issue duplicata #2080 e il commento di workaround referenziato, quindi determina il comportamento previsto di apertura al passaggio del mouse e chiusura quando il mouse lascia l’elemento, sia per l’elemento menu sia per il dropdown; il lavoro è completato quando questo comportamento è integrato come opzione supportata di MdMenu.

Scritto dal modello di indicizzazione a partire dal testo della issue.