Overhaul macOS packaging:

- Build on Travis-CI macOS VM
- Sign and notarize for distribution
- Create macOS-specific files from generic sources during packaging
- Add volume icon
- Rename osx directory to macos

packaging/package-all.sh

@@ -27,9 +27,12 @@ function build_package() (
 
 #exit on any non-zero exited (failed) command
 set -e
-build_package windows
-build_package osx
-build_package linux
-build_package source
+if [[ "$OSTYPE" == "darwin"* ]]; then
+  build_package macos
+else
+  build_package windows
+  build_package linux
+  build_package source
+fi
 
 echo "Package build done."