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

Call GetDatabase() only once.

The proxy plugin has now implemented the "idle" event listener.

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

Reduce GLib usage.

Don't use GTimer if the operating system is able to tell us the
uptime.

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.

Fixes crash on "stats" in certain configurations.

Eliminate redundant declaration.

Unused outside of Stats.cxx.

Publish the Client API, preparing to move more code into the Client
class.

Use forward declaration instead.

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

Replaces GLib's GError.

Optimize the ProxyDatabase by invoking "stats" on the peer, instead of
visiting all songs.

Preparing for non-"simple" database plugins.

Fixes warning on mingw32.

Add nonnull attributes and fix all callers.

Use this struct for db_walk().