Kellyroach/upgrade to xcode10

[kellyroach]

This pull request updates PuzzleMaker to Xcode 10 / Swift 4.2 . Current master branch demo app gets 4 yellow warnings and 2 red build errors in Xcode 10:

4x "using '!' is not allowed here; treating this as '?' instead"
"value of optional type 'PuzzleUnit?' must be unwrapped to a value of type 'PuzzleUnit'"
"cannot convert value of type '@lvalue [[PuzzleElement?]]' to closure result type '[[PuzzleElement]]'"

Our pull request fixes these issues and improves the image size and numColumns / numRows appearance in the demo app a bit.

• Improved .gitignore
• Update to Xcode 10 / Swift 4.2 SWIFT_VERSION = 4.2 in targets Changes to some *.swift to get them to be 4.2 compatible Deployment Target = 8.0
• Demo app resize image / numColumns / numRows for given device.
• Add AppIcon to demo app.

[pwlkania]

Thank you @kellyroach for your support!