[mat-menu] support horizontal (xPosition) overlap detection

[shlomiassaf]

Bug, feature request, or proposal:

feature

What is the expected behavior?

When [overlapTrigger] is false the menu will NOT overlap on the horizontal axis (xPosition)

What is the current behavior?

The horizontal axis is overlapping the triggering element wether overlapTrigger is true/false.

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

Per the docs, the overlapTrigger should respect both x and y.
It is also a valid requirement.

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

angular 5, material 5.0.0-rc0

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