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)