Fix shape padding handles in German locale

Previously handle could not be adjusted in locales with commas as decimal separators. Same fix not needed elsewhere, searching with regex for "g_strdup_printf.+%f", ignoring uses for UI and file-update, which already sets locale to C.

#3240 (closed)

Edit: the gears are slowly ticking in my head that the above regex search isn't good enough (think %3f)

A more thorough search with g_strdup_printf.+%[\d\.+-]*f turns up src/extension/internal/pov-out.cpp (unaffected/commented out, and uses g_ascii_formatd instead) and BasicWriter::writeFloat and BasicWriter::writeDouble in src/io/stream/inkscapestream.cpp

Those BasicaWriter calls are currently unused and I'm a bit hesitant to touch them. If you were, you'd probably use g_ascii_formatd though