mdPanel - integrate with $mdCompiler changes

[devversion]

• Recently we made some refactoring on the $mdCompiler and it now supports contentElements

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Task 1

Rather than manually removing the element from the DOM, the panel should use the cleanup method, to integrate with the contentElement functionality.

const compileData = $mdCompiler.compile(...);

/* Removes or restores the element */
compileData.cleanup();

Task 2

To properly support contentElements, the transition classes need to be reverted before calling the cleanup function.

You can either

• Manually remove the transition classes
• Reversely run the $animate runner.

Task 3

Add documentation to the panel, to let users know how they can use a contentElement.

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This currently blocks the port of the dialog to the panel (#9675).