MSVC support

Creating this issue to keep track of tasks required to make LMMS build compatible with MSVC (related to #3492). I'll try to keep this list up to date:

- [x] Fix LMMS core code & CMake: Done via #4000
- [ ] Fix plugins that fail building:
- [x] LadspaEffect: done via #6973
- [x] Monstro: done via b7c09974acc3644e6aaf1324c853f77cec152efa
- [X] Organic: done per ac65fb99245b44655ef7ac1f637bb2a68808683a
- [x] ReverbSC: done via 57c85f49b7e0675a9a8704e4673543fedfd5eb96
- [x] SID: done via a0ace86
- [x] VST: done via #4421 and f245d7454b9e0c07959f804e046d80d489666db7
- [x] Vibed: done via adcc009d2bda519bb62365121e6d8f1769f61a09
- [x] Xpressive: done via 1c5e374192da80d09b8c4078c316bc0de4214a01
- [x] ZynAddSubFX: done via #6561
- [x] Mallets: done via #6507
- [x] SF2 player: done via #6507
- [ ] Gig player: in progress at #7162
- [x] Add continuous integration, options: (done via #4439):
* [AppVeyor](https://www.appveyor.com/)
* [Visual Studio Team Services](https://www.visualstudio.com/de/team-services/continuous-integration/)

~Because Windows provides no package management that we can use for grabbing dependencies, we'll need to find another way of making the overall build process easier.~
[VCPKG](https://github.com/Microsoft/vcpkg) is gaining popularity quickly and already includes almost all of the dependencies we need. The only issue with it is that it always compiles from source, so it's not a good candidate for huge dependencies like Qt. It can be used together with the official Qt binaries though.

*Edit:* Maybe GitHub's "Project" feature is better for this?

Start by reviewing the unchecked Gig player task and issue #7162, then compare it with the completed plugin and CI entries in this checklist. Check the MSVC/CMake build path and the AppVeyor or Visual Studio Team Services configuration mentioned here; done means the remaining Gig player work is resolved and MSVC build coverage passes.

Written by the indexing model from the issue text.