Use maps instead of vectors for holding strips and plugins
This allows to deal with "holes" after objects are removed
Also paves the way for a future improved way of identifying
individual strips and plugins
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.
"Always use Tabstops for block-indent (the code must be
formatted correctly with "[TAB] = N spaces" for any value of N).
Use space only for alignment." - https://ardour.org/styleguide.html
