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/CMakeLists.txt

@@ -22,6 +22,7 @@ set(SLIC3R_GUI_SOURCES
     GUI/ConfigSnapshotDialog.hpp
     GUI/3DScene.cpp
     GUI/3DScene.hpp
+    GUI/format.hpp
     GUI/GLShader.cpp
     GUI/GLShader.hpp    
     GUI/GLCanvas3D.hpp