A new `env.Run` method is added which allows to control the verbosity
of builders output automatically depending on whether the "verbose"
option is set. It also allows to optionally run any SCons commands in a
subprocess using the existing `run_in_subprocess` method, unifying
the interface. `Action` objects wrap all builder functions to include a
short build message associated with any action.
Notably, this removes quite verbose output generated by `make_doc_header`
and `make_editor_icons_action` builders.
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