Allow soundfonts to replace existing

Nobody has claimed this yet.

### Enhancement Summary
Allow drag & drop soundfonts into existing instance of SF2 Players GUI window to replace
current soundfont with a different one.
(Really just looking for upgrades that make testing many soundfonts out more efficient, I will be making posts that revolve around this desire)
(Using Linux based AppImage version of LMMS)
Was wondering if anyone would be interested in making the following update.

### Implementation Details / Mockup
`DESIRED BEHAVIOR`
When you already have an instance of the SF2 Player inserted in your project, that already has a soundfont loaded in it.
Allow the ability to be able to open open the SF2 Player GUI window and be able to drag and drop a different soundfont anywhere in the SF2 Players GUI window and have this new soundfont replace the existing one.
Would like this to be able to be drag & dropped anywhere in the GUI window, would NOT be desired to have to D&D soundfont into a tiny little path field or something like that.

`AFP EXAMPLE`
To give an example this would give similar functionality to what the AFP offers with samples.
You can open AFP GUI window and D&D audio files anywhere in the GUI and it replaces the existing audio file with a new one.

`BENEFITS`
The idea is to make it easier to be able to test out many different soundfonts as efficiently as possible. There are tons of soundfonts available would be nice to have some built in ways of testing them out more effectively.

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I made another post regarding the SF2 Player that also has an idea linked to this one if anyone is interested. Can be found here if anyone is interested #7481

Thank You

### 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.

Start by locating the SF2 Player GUI and the AFP drag-and-drop handling referenced in the issue, then trace how the current soundfont is selected and replaced. Done means dropping a different soundfont anywhere in the open SF2 Player window replaces the loaded soundfont without requiring the path field, with existing behavior preserved.

Written by the indexing model from the issue text.