Display correctly in Mojave dark mode

[scottmmjackson]

TodoTxtMac generally works correctly in dark mode except that the todo entry field is drawn with a white background (and the "default" text color). This works fine in light mode, but it means that text is invisible when you type it.

I changed the custom text field to use NSColor.textBackgroundColor as recommended by Apple. See:

• https://developer.apple.com/documentation/appkit/supporting_dark_mode_in_your_interface
• https://developer.apple.com/design/human-interface-guidelines/macos/visual-design/dark-mode/

[scottmmjackson]

Here's a sample of the new look:

[scottmmjackson]

Hey @mjdescy - could we get a merge and a new release? I love this application and just want it not to be the brightest thing on my screen. Thank you!