Client window mixer scrollbar

**Describe the bug**
The client main window is allowed to get small enough that the mixer panel shows a scrollbar.

**To Reproduce**
* Connect to a server
* Reduce the client window size down:
* Continue reducing the window size down.

**Expected behaviour**
The window should not get smaller.

**Actual behaviour**
The window gets smaller, causing a vertical scrollbar.

**Operating system**
Windows

**Version of Jamulus**
Version 3.7.0dev-74f55f2

**Additional context**
I don't remember when it started happening but it was probably in 3.7.0 or shortly before.

Start by reproducing the issue on Windows by connecting to a server and repeatedly shrinking the client main window, then trace the code responsible for the mixer panel and window sizing. Done means the window stops shrinking before the mixer needs a vertical scrollbar.

Written by the indexing model from the issue text.