UX: Client Settings Profile Instrument List

**Describe the bug**

The "Instrument" drop down is harder to use than it needs to be.

**To Reproduce**

Run Jamulus Client with the GUI.

Open the Settings, My Profile tab and click the "Instrument" drop down.

**Expected behavior**

The list of instruments should _drop down_ from the control, with the selected entry still under the cursor. (This is how the beta1 release - from the installer - works on Windows for me.)

**Actual behavior**

The list fills the screen from top to bottom. Finding the curent entry is difficult as it's not necessarily anywhere near the pointer.

**Screenshots**

Click the selected item...
Some other entry is no under the pointer.


**Operating system**

Linux

**Version of Jamulus**

3.9.0beta1 (and various other versions)
**Qt 5.12.8** which could be the cause. The Windows version ships with Qt 5.15.2, whilst the Linux version uses dynamic libraries already installed on the system.

Start by running the Jamulus Client GUI on Linux, then open Settings > My Profile and inspect the Instrument control, comparing its behavior with the Windows beta1 behavior described. Done means the instrument list drops down from the control and keeps the selected entry under the pointer.

Written by the indexing model from the issue text.