Add internal API to set whether the application should show up in the task bar

We need a way for the deploy code to control whether or not the running implementation is a TaskBar application. It will be enabled by default and disabled for applets (and maybe later for other cases). On Mac, if the TaskBarApplication attribute is disabled, Glass will use Cocoa APIs to disable the app showing up in the dock icon.

The initial implementation will be to set a system property at platform initialization time if this attribute is set to false:

    com.sun.glass.taskbarApplication=false