Refactor ExternalPopup

The current code loads a list of KServices, saves only their user-visible names and icons and later reconstructes the KService via KMimeTypeTrader. Instead keep the KService itself around so we can use it later.

The code also connects to the triggered signal of the menu and then uses the user-visible action name to determine what codepath to trigger. Instead connect to the triggered signal of the individual actions with slots for the specific actions to better separate the codepaths.