Slic3r::format(), Slic3r::GUI::format_wxstr():

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().

src/slic3r/pchheader.hpp

@@ -183,3 +183,5 @@
 #include "libslic3r/BoundingBox.hpp"
 #include "libslic3r/ClipperUtils.hpp"
 #include "libslic3r/libslic3r.h"
+
+#include "GUI/format.hpp"