Copy and paste notes between LMMS and other applications

### Enhancement Summary

On desktop OSs like Windows, we can copy and paste text and images between softwares. For example, we can copy an image from Microsoft Word into Adobe Photoshop. However, this isn't currently possible for midi notes.
I hope that we can copy notes between LMMS, sheet music editors (such as Musescore), Singing voice synthesizers (such as OpenUtau), or any other symbol-based music creation software.

### Implementation Details / Mockup

To implement this feature, we should define a common clipboard data exchange format for musical notes.
I'm probably not the first one who come up with this idea, so maybe some softwares already implemented a format. However, they aren't widely supported.
If there is really no such data exchange format existing, ExpressiveLabs has proposed one: https://github.com/ExpressiveLabs/commonnote . You can discuss with them if you're interested or have suggestions, since it is new.

### 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.

No source files, tests, or entry points are identified. Start by researching the proposed commonnote format and how LMMS currently handles clipboard data; define the interoperability requirements and a supported musical-note exchange format before implementation.

Written by the indexing model from the issue text.