Pattern files dragging and dropping

[IoeCmcomc]

Is your feature request related to a problem? Please describe.
Since we can drag and drop .nbs files from Explorer into the program, it would be great if we could do the same thing with pattern files (.nbp).

Describe the enhancement you'd like
When dropping a (vaild) .nbp file, the pattern should be placed at the cursor position instead of the start of the song. If possible, it'd be nice to have the ability to preview the pattern while dragging.

Additional context
None.

[Bentroen]

Added in 4bc4661.