Modify Mac draggable region for Electron 24

[PalmerAL]

In Electron 24, we can't have a region that is both draggable and clickable: https://github.com/electron/electron/issues/37789. This makes it impossible to drag the window on macOS with the existing titlebar design.

Windows and Linux currently have the same limitation; we solve it there by shifting the titlebar down and adding empty space above (see previous discussion: https://github.com/minbrowser/min/issues/630). But no other apps do the same thing on macOS, so it would look odd to do it there.

My current idea (which this PR implements) is to do this, where the draggable regions are in red:

Although the space to the right of the window controls looks slightly odd, and it's not obvious what's draggable:

If anyone has feedback or other ideas, please let me know!

[PalmerAL]

I'm going to merge this and make a beta release to get more feedback, and then we can adjust it again before the final release.