Misc minor code cleanups

Merged Alan Coopersmith requested to merge alanc/xditview:misc into master
  • Variable scope reductions as recommended by cppcheck
  • Use _CONST_X_STRING to make libXt declare String as const char *
  • Clear -Wmissing-field-initializers warnings
  • Fix -Wsign-compare warnings
  • Delete HorizontalMove macro that shadows/duplicates HorizontalMove function
  • Inline *Move & *Goto functions
  • Use standard strrchr() instead of ancient rindex()
  • Avoid leak of new_file if we decide not to use it
  • Declare guesses const

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