GLib.find_program_in_path() is used instead of spawning which

Hello xtranophilist,

this pull request replaces the implementation of _detectHDDTemp that used the 'which' executable to locate hddtemp. The new implementation uses GLib.find_program_in_path instead to follow the idea of commit SHA: 9fe9a4f112070870922dbdc3470553a04b38f918

regards

Marcel Metz