Add Windows compatibility

[minorsecond]

Is your feature request related to a problem? Please describe.
Adding Windows compatibility would round out the 3 leading OS's and would make the app available to more people.

Describe the solution you'd like
Add macros where needed to call Windows only functions. Also, update CMakeLists.txt to perform Windows build actions.

[minorsecond]

We're halfway there. Icons are missing and the calendar arrows aren't there.