Introduce Mojave style tabs:

[balthisar]

• Update minimum macOS requirement to 10.10.
• Adopt non-deprecated API's for this change.
• Xcode version check updated.
• Add Mojave tabs to sample app.
• Sample app uses NSVisualEffectView, just to modernize it and better demonstrate modern concept.

About:

• Dark mode works in 10.14+, and is automatic.
• Nearly faithful reproduction, given some MMTabBarView API limitations.
• Light and Dark Modes.
• Proper appearance when window is not active window.
• High contrast modes.

This is probably the best tab style there is for MMTabBarView, in terms of functionality and adherence to Apple's user interface.

[TAKeanice]

I have not seen a reason to drop 10.9 support in your commit @balthisar . Can you elaborate which modern, incompatible APIs you used? Reverting in XCode to 10.9 threw no compiler errors ("only available starting 10.10" or the like). Additionally, there were some strange @available(macOS 10.11) checks, where the if branch and the else branch were the same. I was able to remove all of them and still compile just fine. Take a look at the version in the Vienna repository for reference: https://github.com/ViennaRSS/MMTabBarView/pull/7

[TAKeanice]

I figured out why there were the @available checks, but the reasons for the incompatibility with 10.9 would still be of interest to me. Meanwhile, in Vienna, we tested our version on 10.9, and it works flawlessly.

[barijaona]

Sorry, @TAKeanice, I caused confusion, due to problems with my builds.

I correct : Mojave style does not work in 10.9. In MMMojaveTabStyle+Assets.m, there are two instances of uses of [NSColor labelColor] which can only be used in 10.10 or later.

[TAKeanice]

Alright, that means the color is incorrect? We can circumvent this with @available checks at the two instances, can´t we?

[barijaona]

It means the app throws an exception. Yes, this could probably be circumvented with @available checks, but I haven't tried it yet.