guess content-type from parent pid

[Antoine Martin]

If we don't find a match for the window using the regular content-type guessing code (#1950), use the parent pid.
Useful for applications like steam: all children processes should use video by default.

See ​Given a child PID how can you get the parent PID and ​Find parent pid given a pid: requires libproc-dev.

#include <proc/readproc.h>
void printppid(pid_t pid) {
    proc_t process_info;
    get_proc_stats(pid, &process_info);
    printf("Parent of pid=%d is pid=%d\n", pid, process_info.ppid);
}

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2753