macOS app bundle example should use relative paths

[parro-it]

moved from discussion #66

[mischnic]

/Users/parroit/.nvm/versions/node/v9.5.0/bin/node /Users/parroit/repos/libui-node/examples/control-gallery.js&

can be replaced by

/Users/parroit/.nvm/versions/node/v9.5.0/bin/node "$(dirname $0)/../../../control-gallery.js"

But the main problem is that node is usually not in the default PATH (/usr/bin:/bin:/usr/sbin:/sbin)

PATH="/usr/local/bin:$PATH" node "$(dirname $0)/../../../control-gallery.js"

would solve it for me (I installed node with homebrew), but not when using nvm.
It there something like ~/.nvm/current/bin/node or nvm path current?

---

A reminder for myself while we're at it:
This shows "Control Gallery" instead of "node" in the menubar.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>CFBundleName</key>
	<string>Control Gallery</string>
</dict>
</plist>