Accessibility: Missing keyboard navigability and focus

[Menelion]

First of all, let me thank you for mentioning accessibility straight in the main Readme file for the repo, it's hugely appreciated. As a screen reader user and a software engineer myself, I'm in constant search of modern accessible solutions implementing native UIs for different platforms. Now to the issue itself.

Steps to reproduce

• clone the repo in Windows.
• Open your Windows Terminal in PowerShell or whatever you use
• Do zig build calculator
• When the calculator is run, try to navigate with your keyboard using Tab or your arrow keys.
  Expected result: The focus should be placed somewhere in the window on first run, probably in the edit field; Tab key should move through the buttons and other controls.
  Actual Result: Nothing happens. As a screen reader user, I can only emulate mouse clicks which is far from being accessible and usable.

[zenith391]

You're right, I'll try to make a fix in the next few hours. Also, while reproducing the bug I noticed keyboard navigation was broken on the Linux backend too so I'll fix that too

[zenith391]

So far I've been able to develop something basic but that works on Windows, but I'll have to expand my solution a bit more to allow support for arrow keys.