Trying to open another project while playing a project

### Bug Summary
Trying to open a project while another project is playing leads to about a fraction of a second of audio repeating until the next project is loaded.

#### Steps to reproduce
1. Open a project and play it
2. While project is playing try to open another project from within LMMS
3. While the second project is loading a fraction of second of first one repeats (audio glitch noises).

#### Expected behavior
Project stops playing audio while next one is loading

#### Actual behavior
Audio does not stop and instead, audio is heard "glitching".

#### Screenshot
Video: (enable volume)

https://user-images.githubusercontent.com/76674645/129730326-32527fb3-362f-43ce-aaf6-26c6b4739ffa.mp4

(The open dialogue box isn't showing in the video because of screen recorder settings. I clicked the "Recent files button" and selected a recently opened file.)
Notice how the audio glitches while the next project is loading.

#### Affected LMMS versions
1.2.0, 1.3 alpha

Start by reproducing the issue in LMMS 1.2.0 or 1.3 alpha: play one project, then open another from Recent files. Trace the project-loading and audio-playback paths to identify where playback remains active; done means audio stops before loading begins and no repeating glitch is heard.

Written by the indexing model from the issue text.