Rename all {object}_n variables and arguments to {object}_id
Parts of code were using the former convention, now use the latter everywhere
Another step towards supporting visual position agnostic identifiers in the future
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
