Add Carla support for Windows

[tresf]

Closes #4654

This PR should be safe for backport to 1.2.x if needed.

What the PR does:

• Renames libcarla_native-plugin.dll to carla.dll to match that of the Windows release
• Fixes a PATH conflict preventing Carla from finding PyQt

Steps to use Carla with LMMS:

• Install a version of LMMS with Carla support (click here if the download bot below doesn't have one)
• Download Carla for windows
• Extract Carla to a permanent location
• Add Carla.lv2 to your PATH environment variable (e.g. C:\Carla_2.2.0-win64\Carla.lv2)
• Start LMMS

Known issues:

• Removing a Carla track may cause LMMS to segfault
• On slow machines, Carla may not load the first time and "Show GUI" may need to be clicked again

[LmmsBot]

🤖 Hey, I'm @LmmsBot from github.com/lmms/bot and I made downloads for this pull request, click me to make them magically appear! 🎩

Linux

• Linux (AppImage): lmms-1.2.3-734%2Bgcf4fd59-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.3-734%2Bgcf4fd599b-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.3-734%2Bgcf4fd599b-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.2-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.2-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.2.3-734%2Bgcf4fd599b-mac10.13.dmg (build link)

🤖

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[PhysSong]

Removing a Carla track may cause LMMS to segfault

Reproduced with Carla 2.2 and MSVC build of LMMS. It seems like Carla crashed in CarlaEngine::ProtectedData::deletePluginsAsNeeded() which is indirectly called from CarlaInstrument::timerEvent(). Inspecting memory and registers suggests there might be some use after free conditions.
2.1.1 Also crashes in CarlaBackend::CarlaEngineNative::uiIdle(), but less frequently.

[PhysSong]

I found that it's related to the (improperly implemented) InstrumentTrackWindow caching which causes a temporary CarlaInstrumentView to be instantiated after the existing one is deleted. That will be deleted after CarlaInstrument, and timer events may kick in between those two events. That results in CarlaInstrumentView::timerEvent() using a deleted handle.

[tresf]

I found that it's related to the (improperly implemented) InstrumentTrackWindow caching which causes a temporary CarlaInstrumentView to be instantiated after the existing one is deleted. That will be deleted after CarlaInstrument, and timer events may kick in between those two events. That results in CarlaInstrumentView::timerEvent() using a deleted handle.

@PhysSong would you like to wait to merge this PR until the bug is resolved, or merge and open a new issue?

[tresf]

@PhysSong would you like to wait to merge this PR until the bug is resolved, or merge and open a new issue?

Merging. I'll open a new issue for the crash.