macOS: updates to version and custom app icon handling

[danomatika]

This PR implements changes proposed in #429 and #430:

• separate Project.xcconfig VERSION into the default MARKETING_VERSION and CURRENT_PROJECT_VERSION variables to match Xcode UI
• add examples to new method for setting custom app icons as well as info in transitioning from previous ICON_FILE_PATH variable

[danomatika]

I was tempted to also change some formatting and add sections like:

// APP SETTINGS #####
...

// VERSIONING #####
// uncomment to override settings in Xcode UI

// this is "Version" in the Xcode target Identity UI
// suggested to use semantic versioning format ala #.#.#
//MARKETING_VERSION = 0.1.0

// this is "Build" in the Xcode target Identity UI, an incremental build number
// important for the App Store as new build submissions need a diff number even
// if MARKETING_VERSION is the same
//CURRENT_PRODUCT_VERSION = 1

// ICONS #####

// default oF app icon
....

... but I held off for now to keep this specific.

I would also add a leading space to comments so:

//THE PATH TO THE ROOT OF OUR OF PATH RELATIVE TO THIS PROJECT.

becomes

// THE PATH TO THE ROOT OF OUR OF PATH RELATIVE TO THIS PROJECT.

... but that is a stylistic difference which doesn't have a huge impact other than my general pursuit of consistency.

[ofTheo]

just fyi @danomatika
the changes above will only affect the Project generator command line app.

To do the same for all OF apps you would need to modify the files here:
https://github.com/openframeworks/openFrameworks/tree/master/scripts/templates/osx

[danomatika]

Why would there need to be two copies of the Xcode project files? In any case, I can do a separate PR to OF tomorrow.

[ofTheo]

@danomatika the ones at: https://github.com/openframeworks/openFrameworks/tree/master/scripts/templates/osx
are what all OF projects and examples use.

The ones in this repo are customized just for building the PG command line app and some of those settings we might not want to be global ( ie: in the same way the individual projects might want to have different Project.xconfig settings ).

[danomatika]

Latest commit uncomments version variables as they act as defaults are are overridden by the Xcode UI. Also, the build variable name is corrected to CURRENT_PROJECT_VERSION.

[ofTheo]

Awesome - thanks @danomatika !!