Add launching capabilities to Applets

[alicerunsonfedora]

Currently, desktop shells are unable to launch Applets as there is no way to definitively tell these shells what action will occur when launching them. To solve this problem and give desktop shells this functionality, I've added a new set of properties to the Applets framework:

# Define how your Applet will act with desktop shells.
launch = {
    "action": "Return(0)", # The action that happens when a user clicks the button
    "show_in_launcher": False # Whether it should appear in desktop shells
}

Update: Applets that act as desktop shells must include the pm_shell permission.