Replaced the single column icon with an empty icon.
Will it break something? The single column added an ugly spacer
in front of non-editable combo boxes.
Removed libslic3r from the default include paths for all modules but
libslic3r. Now headers from libslic3r need to be included with an
explicit path (libslic3r/libslic3r.h etc)
Split the localization macros into I18N.{cpp,h}