Need a mechanism for specifying an ical event's icon

[Charles Kerr]

This issue was raised in discussion with mzanetti, who would like reminders (created with reminder-app) to be shown in the drop-down menu with that app's icon.

datetime currently is implemented with "everything is either a clock-app alarm or a calendar-app event", which is not an approach that will scale to 3rd party apps. I suggest we add something like an x-unity-alarm-icon tag to the ical components to let clients specify their own icon as a string that gets passed to unity8.

See related bug https://bugs.launchpad.net/ubuntu/+source/indicator-datetime/+bug/1426519 for letting the clients, rather than datetime, specify the behavior when the menuitems are clicked.