GDK W32: Fix property setting to correctly use static strings

The strings that are set are static, so g_value_set_static_string() should
be used to set them instead of g_value_set_string().
2 jobs from ime-load-fix in 8 minutes and 9 seconds
Status Job ID Name Coverage
  Build
passed #250231
build

00:08:09

manual #250232
win32 allowed to fail manual
msys2-mingw32