Move Timing into separate context

Move Timing into separate context

The advantage over an alias at Score level is that setting Timing
variables before a Score context is established does not lead to error
messages.

Having a real, mostly ignored Timing context as a subcontext of Score
is a bit ugly, though.

An alternative would be to establish a Timing alias for Global instead
since Global is always present.  It would be somewhat unusual to
manipulate Global variables, however.

It can't be an alternative to actually _accept_ Timing at Global level
since get_score_context relies on Score being the only subcontext of
Global.

The last commit concerns just ly/engraver-init.ly and ly/performer-init.ly

There are two additional commits:


Make Timing_translator do a more thorough job of initializing Timing

Touches lily/timing-translator.cc


Replace base-fraction with base-length to simplify calculations

touches scm/define-music-callbacks.scm and scm/time-signature-settings.scm

[dak, 2013-01-25 16:37:55]

Timing context in performer-init.ly belongs to Performer_group

[dak, 2013-01-26 09:44:12]

Without creating the Timing context, there will be nothing to be found