Warn on installer/uninstaller if LMMS is open

### Enhancement Summary

Currently the installer/uninstaller will just plow through the install directory, not knowing that some files are locked by the open program and will hang or error the process.

It would be nice if the (in/unin)staller can close or warn about this before it starts file operations, which will lead to a corrupted install or an unfinished uninstall

### Implementation Details / Mockup

* https://stackoverflow.com/questions/2565215/checking-if-the-application-is-running-in-nsis-before-uninstalling

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Locate the installer and uninstaller definitions, then review the linked NSIS example for detecting an open LMMS process. Define the behavior before file operations: warn or close LMMS, and verify that installation and uninstallation do not hang or leave incomplete results when LMMS is open.

Written by the indexing model from the issue text.