The new method IsPlugin() replaces the "is_simple" flag.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Don't use the global variables from the DatabaseSimple library.

Allow database plugins to announce that they have been modified.

Ask the DatabasePlugin for stats when the first client requests them,
not at startup.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

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

Replaces GLib's GError.

Use the C++ API.

RTTI adds overhead to the binary, and we don't need that.

Allow the plugin to allocate the GetSong() return value.

No more hard-coded "simple" plugin.

Let the caller take care for legacy conversion.

Preparing for non-"simple" database plugins.

Unused.

Allow using the object outside of DatabaseGlue.cxx.