Move custom/macos* files to macos/*

Move all macOS build scripts and resources into the same directory instead of
just having the scripts in one directory and the resources in another.

Makefile

@@ -1033,14 +1033,14 @@ install-macbundle:
 	@install -m 644 macos/PkgInfo $(BUNDLE)/Contents
 	@install -m 644 macos/Info.plist $(BUNDLE)/Contents
 	@install -m 644 macos/tuxpaint.icns $(BUNDLE)/Contents/Resources
-	@custom/macos.sh
+	@macos/build-app.sh
 
 
 # Create DMG for macOS
 TuxPaint.dmg:
 	@echo
 	@echo "...Creating DMG Distribution File..."
-	@custom/macos-mkdmg.sh
+	@macos/build-dmg.sh
 
 
 # Build the program!

custom/README.txt

@@ -2,8 +2,5 @@ Tux Paint Customization
 
 This folder allows for customization of Tux Paint. One example is adding resources such as fonts, stamps, starters...
 
-On Mac OS X, at the very end of the build, macos.sh script in this folder is run.
+On Windows, the win32.bat in this folder has to be run manually. At some point, a calling to the script could be integrated in the build process so as to be run automatically.
 
-On Windows, the win32.bat in this folder has to be run manually. At some point, a calling to the script could be integrated in the build process so as to be run automatically, just like it is on Mac OS X (see above).
-
-On Linux, it should be straightforward to replicate the process describe above on Mac OS X, and automatically call a linux.sh script, in this folder, from the build process.