This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Avoid GLib.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Look up top-level config options by enum (= integer), not by name
string.