feat(panel): Configuration ID for tracking

[bradrich]

Actual Behavior:

• What is the issue? * Currently, the mdPanel service assigns an ID, that is a simple string, to any panel that it creates. However, the service does nothing to track the panels with this ID. It is possible to open multiple panels by clicking on the same button over and over.
• What is the expected behavior? The user should be able to provide an ID within the panel configuration. When an ID is provided, the created panel is added to a tracked panels object. Any subsequent requests made to create a panel with that ID should be ignored.

CodePen (or steps to reproduce the issue): *

• CodePen Demo which shows your issue: N/A
• Details: N/A

Angular Versions: *

• Angular Version: Latest
• Angular Material Version: Latest

Additional Information:

• Browser Type: * N/A
• Browser Version: * N/A
• OS: * N/A
• Stack Traces: N/A

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