Indicator should detect when Clock and Calendar are installed/removed

[Ted Gould]

On the Phone the indicator displays both menu items for the Calendar and Clock. It is possible that each of these could be installed or removed during the lifecycle of the indicator. It should respond appropriately and remove or disable the menu items if there is no clock or calendar to be launched.

This can be achieved by setting up a click hook to execute on install/remove of packages so that it can respond appropriately.