Load additional actions for app entries in application launchers

This allows 3rd parties to provide additional actions in the context
menu of applications in kicker, kickoff,... by supplying suitable
desktopfiles in GENERIC_DATA_LOCATION/plasma/kickeractions similar
to mechanism used for service menues.
Entries for which the actions are displayed can be controlled by
X-KDE-OnlyForAppIds in the [Desktop Entry], for example
X-KDE-OnlyForAppIds=org.kde.kate,org.kde.dolphin
This was requested as a patch for steam, but we've made a generic
solution so that it can be used by everyone.