Windows: Add terminal support

[jgillich]

Summary

Add support for the exec terminal on the Windows platform.

Motivation

Feature-parity with UNIX version.

Proposal

We use vte for the exec terminal, which is not available on Windows (this is unlikely to change). We should aim for feature-parity across all platforms, therefore we need a different solution for Windows.

I had started an implementation using Xterm.js and webkitgtk until I realized webkitgtk's Windows support is iffy too. I'm not sure what the best path forward is.

1. Spawn a Powershell window with kubectl? Not very nice.
2. Implement a Xterm.js-equivalent using a simple widget like a textview? Likely a huge amount of work.
3. Attempt to integrate native go solutions such as webview with GTK (not sure if this is possible) and use Xterm.js
4. ??