VST3 Support

Hello,
I'm re-opening issue [#2352](https://github.com/LMMS/lmms/issues/2352)
It was from 2015, it was a copy-paste, and today, in 2018, the context is different.

**Context :**
On the 1st Oct 2018, Steinberg stopped suporting VST2.
What does that means ? (The way I understand it, based on [this topic](https://forum.juce.com/t/steinberg-closing-down-vst2-for-good/))
- For devs who have a license : they can freely release new VST2 plugins.
- For devs who don't have a license, unfortunately, they are not allowed to distribute VST2 in any ways, and they CAN'T ask for a license.

Since may, I'm working on a big project, a free EDM synthesizer, using Cabbage framework..
Unfortunately I can't release it as a VST2, because I'm too late in the game...

The only way I will be able to distribute it is by the form of a VST3, [which support GPL3 licensing](https://sdk.steinberg.net/viewtopic.php?t=286).

**Request**
- Implementation of VT3 support for LMMS

I think it would be a really good improvement for LMMS, and I'd be pleased if my plugin could be compatible with the DAW I use.

Unfortunately, my knowledge to add this support is I think, too limited to lead the project, but I would be pleased to help & learn, so if you guys have knowldge about it, feel free to share it here so I can learn how it works in the same time (I know some devs have solid knowledge on this field, and would be interested to implement VST3 support for LMMS)

Thanks in advance,