IsSettingSaveable.cpp: Significant code size reduction.

- Refactor the Config::System::Main check so we check system once,
  then we check for the section.

- Use an std::array<> instead of std::vector<>.

- Use an array of pointers instead of an array of ConfigLocation.
  The latter contains two std::string objects, whereas pointers
  are only 8 bytes (on 64-bit).

Code size comparison: (64-bit Linux, gcc-9.2.0, release build)

   text    data     bss     dec     hex filename
  16136       0      40   16176    3f30 IsSettingSaveable.cpp.o [before]
   3933     720       0    4653    122d IsSettingSaveable.cpp.o [after]
 -12203    +720     -40  -11523   -2d03 Difference

0 files changed, 0 insertions, 0 deletions