misc/gedkeeper: the port had been improved (+)

- Make GetTempDir() function Unix-aware, where TMP environment
  variable is preferred over Windows' TEMP, and fallback to /tmp
  when it is unset
- Change GetAppPath() function to uphold that we do not install
  the main program in another subdirectory (bin) since the whole
  project already lives under $DATADIR (Mono/.NET programs are
  not always hier(7)-conforming)
- Fix provided *.desktop file (respect PREFIX and replace badly
  named startup wrapper script with ours)