[MdSelect] Adjust position when body has margin

[AlbertLucianto]

This PR is a possible fix for issue #1206.

Description

This issue might be related to Popper.js library.

After render, popper instance which wraps MdMenuContent is mounted as a direct child of body, and changes in body's position affects where the popper is mounted.

By adjusting popperSettings.modifiers.offset with body's position relative to the viewport, the menu will be correctly positioned.

Screenshots

Style

body {
  margin-top: 100px;
  margin-left: 100px;
}

/* Or an element which can affect body's position */
#app {
  margin-top: 100px;
  margin-left: 100px;
}

Before

Menu is off position the size of the body's margin (or any elements pushing the bodys position).

After

Really appreciate for feedbacks and reviews.

[marcosmoura]

Sounds good for me! Thank you so much!