Desura compatibility

- create launch scripts for Desura
- that access local installation launch scripts
- clean up /usr/* launch scripts created during make install

Makefile

@@ -270,6 +270,9 @@ install: all
 	@$(INSTALL_PROGRAM) -m +rx openra $(BIN_INSTALL_DIR)
 	@$(INSTALL_PROGRAM) -m +rx openra-editor $(BIN_INSTALL_DIR)
 
+	@-rm openra
+	@-rm openra-editor
+
 uninstall:
 	@-rm -r $(INSTALL_DIR)
 	@-rm $(DESTDIR)$(bindir)/openra