Add resource install directory

To fix the relative-path issue when loading textures files, and to
comply with general standard practices, this commit adds changes to the
Makefile to store resources (i.e. texture bitmaps) in the path
'/usr/share/jegChess/'.

The texture bitmaps specifically are stored in an 'img' subdirectory, and
the code in chess-screen.c used to load the textures was changed to reflect this.