[FEATURE] Support drag-and-drop to load .nam models

[jamesWalker55]

Is your feature request related to a problem? Please describe.
In the plugin, the only way to load a model is to use the OS's built-in file selector. However, the location it initially shows seem to reset occasionally. I have to navigate all the way to the folder where I store my *.nam models again whenever this happens.

Describe the solution you'd like
Allow dragging and dropping *.nam models directly onto the plugin, e.g. from a file explorer or the DAW's sample browser.

Most DAWs like FL Studio or Reaper have a built in sample/file explorer and allow you to add 'favourite' folders. It's much easier to browse for .nam models there rather than the OS's built-in file selector, so I would like to suggest allowing dragging directly from an outside source to the plugin.

Describe alternatives you've considered
N/A

Additional context
N/A

[DivannKokos]

at least in windows there are pinned folders in file explorer
and os's built-in file selector in windows is file explorer

[sdatkinson]

Can possibly implement using IControl.OnDrop().

[synchu]

Right, I did a few quick changes here and there and it appears to work (at least on Windows, I have no means to test it with Mac).

@sdatkinson - I can see there's a Pull request that has been posted, so not sure how to proceed.
Do I put a separate Pull request or try to get along with this one (not quite sure of my skills with Github :) )?

[sdatkinson]

@synchu you can start by pushing your changes to a branch on your fork and share about it here.

I (or others) can have a look if I find time and handle the PR. But if you do that then anyone else can merge your changes into a branch of their own 👍

[synchu]

Thanks @sdatkinson. I applied the changes here:

https://github.com/synchu/NeuralAmpModelerPlugin

Now there's a required change into IControl.h, which I am not sure got committed, but it looks like these.
From:
virtual void OnDrop(const char* str) {}
to

virtual void OnDrop(const char* str)
 {
   if (OnDropFunc)
   {
     OnDropFunc(str);
   }
 };

 std::function<void(const char* str)> OnDropFunc;