Global idle events that should partition specific

[jcorporation]

As mentioned in #1521 I open an issue for the remaining global idle events that should be partition specific in my opinion.

Output of git grep idle_add:

src/output/OutputCommand.cxx:   idle_add(IDLE_OUTPUT);
src/output/OutputCommand.cxx:   idle_add(IDLE_OUTPUT);
src/output/OutputCommand.cxx:   idle_add(IDLE_OUTPUT);

I am much too unfamiliar with MPD to prepare a pull request therefor I open this issue.