Improved representation of the MS Basic sound font in the mixer

My idea

1. I propose to consider the location of the MS Basic sound font as a separate (along with Muse Sounds) sound font, excluding it from the SoundFont list (in the Mixer) and placing it in a prominent place, and rename the SoundFont list to "Custom sound Fonts".
2. I propose to allow users to keep the MS Basic list open after selecting/replacing an instrument during parallel playback (i.e. to make sure that the MS Basic list does not close immediately after the user clicks on the instrument in the drop-down list).

Problem to be solved

1. Removing the MS Basic sound font to a separate cell in the mixer will allow users to quickly find the basic sound options they need for certain instruments. Currently, MS Basic is "hidden" among many other sound fonts (if any), and in order to view it, you need to spend some time searching for it. On top of that, the list of SoundFonts is very slow to open if the score is voluminous.
2. Holding the MS Basic drop-down list in the mixer will make it easier for users to select the appropriate sound options for it. Currently, the MS Basic drop-down list is immediately closed after the user clicks on the instrument. This is inconvenient, and the user will have to re-open the list if the instrument he chooses does not suit him. By the way, this applies not only to MS Basic, but also to other custom sound fonts (including Muse Sounds).

Supporting files, videos and screenshots

https://github.com/musescore/MuseScore/assets/134041121/e01de380-75b7-4f6c-a3aa-4354953b9728

Checklist

✅ This request follows the guidelines for reporting issues
✅ I have verified that this feature request has not been logged before, by searching the issue tracker for similar requests

Start by locating the Mixer UI entry points for the MS Basic and SoundFont lists, then reproduce the current behavior using the linked video and a voluminous score. Done means MS Basic has a prominent separate control, the remaining list is labeled "Custom sound Fonts," and instrument selection keeps the relevant list open during playback.