Remove custom sidebar styles

[nth-chile]

Done:

• The main goal of this PR is to remove the custom styling from the sidebar so that it will fall back to system UI
• Now if you press the right arrow key to move to the note list, the sidebar item (on dark mode, at least) will go from blue to grey (selected to unemphasizedSelected). I like this because it shows user where the focus is.

Would be nice:

• I tried to set the sidebar background to NSVisualEffectView.Material.sidebar, which is the slightly transparent material that Mac app sidebars use nowadays (like xCode), but I couldn't get it to work. Maybe there is a background on the ViewController or something that is blocking it?

⚠️ Todo before merging:

• On light mode, text color is black even when selected. Is there a custom style that's keeping it black? Maybe we need to set it to .selectedTextColor
• Icon is blue when cell background is blue. Is there a system icon that will automatically change color when selected?

[nth-chile]

The idea is to remove custom styling so we fall back on defaults as much as possible. When we use colors, we can use system colors like NSColor.textColor this will ensure that as macOS evolves, fsnotes will continue to match the appearance of macOS

@glushchenko do you like this idea? If so, I can fix up the todos here, and can make another PR to do the same thing for notes list

[glushchenko]

I can't test the UI in macOS 10.14, and that has a lot of caveats. It's best not to do that, sorry.