xprop 1.2.6

• Fix spelling/wording issues
• gitlab CI: add a basic build test
• Build xz tarballs instead of bzip2
• gitlab CI: stop requiring Signed-off-by in commits
• Mark help(), usage(), and grammar() with _X_NORETURN _X_COLD
• Declare help() and print_help() as static
• Rename "dpy" function arguments to "display" to avoid shadowing global
• Resolve many gcc -Wsign-compare warnings
• Variable scope reductions as suggested by cppcheck