... Generalizing what was first done at 280e8d9 for certain menu command
handlers.
Allow core data structures to host data attached by other code, on which it
need have no compilation or link dependencies.
... which makes Project.cpp a bit less dependent on some details of other
classes
This puts Tags.cpp back into the big strongly connected component of the
dependency graph. That will be remedied later when Project.cpp becomes a
low-level file
... Send an event via the app that projects listen to for theme update; don't
push it to projects directly.
This frees three files from dependency cycles.
... There was no reason to store the help location preferences as per-project
state. Also move the dialog for quick fixes near its only use in
HelpMenus.cpp.
This takes 22 files out of the big strongly connected component, notably
the much used lower level utilities, ErrorDialog and AudacityException.
HelpSystem itself is still in a small cycle with LinkingHtmlWindow.
... and break its compile dependency on CommandManager.h by letting it install
callbacks.
This also removes Objective-C mixed code from CommmandManager.
This also eliminates four inclusions of Project.h!
Capture handler state is also global, not per project, though the
CommandManager's callbacks still do depend on the active project.
... Reimplement the ESC key handling in TrackPanel and time ruler on all
operating systems so that it does not rely on the focused window, but instead
uses the application-wide event filter.
This includes reversion of 9491605cfc8a7d60117365884fd494996b5ebbaf
... instead define new base classes in src/commands that those other
windows can inherit. The classes have nothing but a virtual destructor. This
lets CommandManager use dynamic_cast to detect the special windows.
See commits cbfde23 and 68e4bf6 which added the #include directives
... for wxString and wxArrayStringEx, holding file paths (absolute or relative,
directory or plain file); to be replaced later with different types
(not yet using std::vector, becase of some uses of wxArrayString::Index with
two arguments)
