RemoteVstPlugin becomes a zombie after crash

### Bug Summary
When LMMS crashes, the ``RemoteVstPlugin``'s do not exit and become zombie processes. To make matters worse, the memory usage seems to slowly trickle up after some time, with the only solution being to terminate ``wineserver``. I'm unsure as to whether or not the way LMMS crashes influences this bug.

#### Steps to reproduce
1. Create a project that has some third-party plugin as an effect on one of the mixer channels.
2. Get LMMS to crash (can be done by sending the ``SIGKILL`` signal to LMMS).

#### Expected behavior
Once LMMS crashes, the ``RemoteVstPlugin`` processes should close, and the memory should not slowly increase over time.

#### Actual behavior
Once LMMS crashes, the ``RemoteVstPlugin`` process stays open and the memory usage slowly increases over time.

#### Affected LMMS versions
Unsure.

Start by locating the RemoteVstPlugin process lifecycle and its cleanup or parent-death handling. Reproduce the issue with a project containing a third-party effect, then send LMMS SIGKILL and observe the plugin processes and memory usage. Done means the RemoteVstPlugin processes exit after LMMS crashes without continued memory growth.

Written by the indexing model from the issue text.