Reduce global variable usage, move to frontend code.

Reduce overhead.  LightSong can always point to the "real" allocated
string.

Playlist file name first, to be consistent with the other functions in
this library.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

This is already being done by FixSeparators(), called from
PathToUTF8().

Don't use global variable.

Useless information.

Call GetDatabase() only once.

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

Reduce overhead, don't duplicate the string.

Reduce overhead.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

Allows the kernel to combine timer wakeups with other processes,
reducing power usage.

Don't use global variables.

Decouple command/OtherCommands.cxx from the "simple" database plugin.

Don't depend on the "simple" database plugin.  This fixes an assertion
failure / crash and allows using stickers with other plugins.