Provide a simple calculator UI (no calculation logic)

[bikathi]

Hi devs, this commit includes code to create a calculator UI using Floem (Git version). This is just a mock UI where there is no calculation logic.
Included are:

1. Grid layouts (for the buttons)
2. Theme switching
3. Flex layout
4. Icons
5. Some window properties (e.g. no maximize)

Here is an example screenshot in dark mode:

[jrmoulton]

Thanks for the PR! sorry for not reviewing this sooner. Could you update this to use the new floem default theme?

[bikathi]

Hi. And sorry from my end for not replying sooner. My intention of the PR was to demo to people how to change themes so I believe if I make the change you're requesting, I'd have lost that goal.

[jrmoulton]

I see. I'll take a closer look :)

[bikathi]

I see. I'll take a closer look :)

Hi. Any update on this?

[jrmoulton]

I haven't had a chance to take a look yet. sorry for being slow to review this. There are a few big changes happening right now but I will review this soon