applets/mediacontroller: fix broken actions
Broken in the Actions refactor when these invocations of the old API weren't ported.
A simple fix would be to port these old invocations of the removed
root.action_thing() function calls to use root.serviceOp() instead, but that
would lead to some code duplication, so instead let's add new top-level
public functions for these things and reference those functions
consistently in all places, which reduces redundancy and fixes the bug automatically.