This fixes several callsites that were passing samplepos_t to get a TempoMetric,
some of them somewhat significant (e.g. VST plugins that want tempo information).
Bad API design on my part, apologies.
This commit combines libs/ and gtk2_ardour because the new private status
of the ::metric_at() call would be a blocking point for git bisect
This commit leaves two issues outstanding:
1. unclear/ugly semantics for drag operations that reset the GUI thread's tempo map to the writable copy
2. undo/redo for the tempo map
These will be addressed in future commits
Terminology used by server and client was starting to diverge.
C++ classes ArdourStrips and ArdourGlobals classes have been
renamed to ArdourMixer and ArdourTransport respectively.
State node values for transport functionality have been simplified
and prefixed with 'transport_' to match what was done for strips.
