Merge latest development work (update to Qt5, 3.2+ shaders) + CMake / build fixes + CI

[past-due]

This extends #4, and merges the latest development work from @inodlite's fork (updating to Qt5, 3.2+ shaders, lots of bug fixes).

It also:

• Fixes CMake builds
• Adds support for building on Windows, macOS, and Linux (via CMake)
• Replaces GLee with GLEW
  • (GLee hasn't been updated in ages, and has issues compiling on some systems.)
• Adds configure scripts for Windows, macOS, and Linux
• Adds a basic README
• Fixes compilation issues on different setups
• Adds CI for Ubuntu 18.04, Fedora (latest), macOS, and Windows / MSVC.

[past-due]

@inodlite: The rebase and merge has updated all the commit hashes. This includes all of the changes from your fork's master branch through https://github.com/inodlite/WMIT/commit/3337bc04b71661b614a12806f45ce44c60b64b00, and then my additional fixes.

(So you may want to rebase your fork, for ease of future PRs / merging. At the moment, this repo's master branch is a superset of your master branch.)