mdIcon: SVGs always focusable in IE

[Fry74]

Issue
In Internet Explorer 11 it appears that SVG icons are always treated as focusable unless explicitly specified otherwise, unlike in Chrome and Firefox. This means that in IE, focus will land on them when tabbing through a document.

Because of this, when mdIcon is used inside other elements (eg. buttons), it takes two presses of the tab key to move focus to the next element.

This makes life more difficult for screen-reader users, as is harder to traverse the document and when an icon gains focus there is no context to indicate what is being focused on.

To Reproduce
For a demonstration of this, view the Angular Material homepage in Chrome and in IE, and tab through the page, paying particular attention to the four large buttons on the homepage ('Getting Started', 'Demos', Customization', 'API Reference'). Notice that in Chrome, pressing tab immediately moves the focus to the next button, where as in IE, it takes two presses to move to the next item. This is because IE is first focusing the SVG icon within the button.

Suggested Fix
SVGs can be made non-focusable by setting them as:
<svg focusable="false>
This could be set on the SVG element when created by mdIcon.