Fixed data install dir and data dir lookup

Previously the openxcom data was installed in /usr/share/openxcom/ and
not the correct /usr/share/openxcom/data/ dir. This is fixed.

You can now also have data spread out in the valid list of data
dirs. For example the openxcom data files in /usr/share/openxcom/data/
and the original xcom datafiles in ~/.local/share/openxcom/data
Merge pull request #466 from DarkDefender/datafolderfix

Fixed data install dir and data dir lookup