Update window layout to work on macOS 10.14 (Mojave)

[robinstewart]

It took me a while to figure out what was going on in the view hierarchy, causing the clock to not get displayed on Mojave. Ideally I'd like to move to an approach that puts the clock in a more standard location in the view hierarchy that would be more robust to OS updates (such as contentView). But after experimenting for a while, I don't have any clear idea of how to do that while maintaining the visual design and behavior.

So instead I was able to adapt the approach for Mojave by simply making the clock view a sibling of the close button.

I also simplified a few lines of code and added comments.

[iossid]

@robinstewart Found an issue, please take a look at the image. Nice try though, I'm playing with the Window thingie too.

[robinstewart]

@iossid When I built against the Mojave frameworks, the window UI didn't show up for me at all, so this pull request just tweaks how the clock views are inserted into the window so that the app works again.

Yes, the transparency issue remains, but I haven't found a fix for that and I figured it could be a separate patch when someone does figure that out.

[iossid]

@robinstewart Yeah, indeed.

[robinstewart]

I realize now my description wasn't clear — my intention with this pull request was not to fix all of the bugs when running on Mojave, but rather to fix the specific, major problem that when the app is built using Xcode in Mojave, the timer window is completely blank:

So this pull request is a necessary first step for any further bug fixes or development.

[karbassi]

@robinstewart Yes, it's completely broken right now. Do you think you could take a look at it?

[robinstewart]

@karbassi Yes, this pull request already contains the fix! It just needs to be merged in.

(I changed the way the clock UI is inserted into the window view hierarchy, which will hopefully be more robust to future OS updates as well.)

[karbassi]

Works for me. Merging!