Correctly guess the assets directory for installed builds

[orbea]

When solarus-quest-editor is installed with -DSOLARUS_INSTALL_BINDIR or -DSOLARUS_INSTALL_DATADIR hard coded assumptions for the install and assets directories will fail. To avoid this add defines for -DSOLARUSEDITOR_BINDIR_PATH and -DSOLARUSEDITOR_DATADIR_PATH instead of -SOLARUSEDITOR_INSTALL_PATH and then remove the hardcoded paths in src/file_tools.cpp.

[christopho]

Thanks!