Set appropriate colours for Multiline Entry control on dark mode (Darwin)

[TropicalBastos]

The PR checks in the following:

• White font colour for multi line entry if user has dark mode enabled
• Dark background colour for multi line entry if user has dark mode enabled
• Utility method for checking if dark mode is enabled
• .gitignore entry for ignoring the build folder

[TropicalBastos]

Only just noticed that there has been a previous PR for this particular dark mode issue. However, it looks like the previous one did not pass all tests.

[LoganDark]

isDarkMode should be either be uiprivIsDarkMode and inside of uipriv_darwin.h, or uiDarwinIsDarkMode and inside of ui_darwin.h (honestly, the latter would be more useful because then people can use it in their libui apps). In both cases it should probably be defined in main.m (or a more specific file) and not a util.m