Move Linux packaging files to contrib/linux

The general idea is to clean up the root directory of the tree
a bit, and to possibly prepare to support flatpak in the future
without adding more files to the root.

Both "make install" and make-rpm.sh worked after these changes.

My idea was also to move src/dosbox.png to contrib/icons/dosbox-x.png
And to move any Windows and macOS specific packaging files to
contrib/windows and contrib/macos respectively, but I will not do the
latter two myself as I cannot test it.

This is also similar to how dosbox-staging has it's tree layout.

If there is no objection, I will however move src/dosbox.png and
update any references to it to point to the new location.

Makefile.am

@@ -78,9 +78,9 @@ install: src/dosbox-x
 	mkdir -p $(DESTDIR)$(prefix)/share/icons/hicolor/48x48/apps
 	install -m 644 src/dosbox.png $(DESTDIR)$(prefix)/share/icons/hicolor/48x48/apps/dosbox-x.png
 	mkdir -p $(DESTDIR)$(prefix)/share/applications
-	install -m 644 dosbox-x.desktop $(DESTDIR)$(prefix)/share/applications
+	install -m 644 contrib/linux/dosbox-x.desktop $(DESTDIR)$(prefix)/share/applications
 	mkdir -p $(DESTDIR)$(prefix)/share/metainfo
-	install -m 644 dosbox-x.appdata.xml $(DESTDIR)$(prefix)/share/metainfo
+	install -m 644 contrib/linux/dosbox-x.appdata.xml $(DESTDIR)$(prefix)/share/metainfo
 	-test -x /usr/sbin/setcap && setcap cap_net_raw=ep $(DESTDIR)$(bindir)/dosbox-x
 
 install_strip: src/dosbox-x install

configure.ac

@@ -1089,7 +1089,7 @@ src/shell/Makefile
 src/platform/Makefile
 src/xBRZ/Makefile
 include/Makefile
-dosbox-x.spec
+contrib/linux/dosbox-x.spec
 make-rpm.sh
 ])
 AC_OUTPUT