wrappers around boost::format using C++17 variadic templates,
replacing the ugly and verbose
(boost::format("template") % arg1 % arg2).str()
syntax.
The wrappers also implictely convert input parameters including the template
from wxString to UTF8.
The new format wrapper has been applied at multiple places as a start,
also some double macros _(L()) with new single macro _L().
