Fixed crash due to incorrect localization of the percent sign in
"The %1% infill pattern is not supposed to work at 100%% density."
This happens for ES, FR, IT, UK and ZH_TW localizations.
Also added a new CMake target "gettext_po_to_mo" to convert
all po files to mo files,
and renamed the "pot" target to "gettext_make_pot".
