Add drag & drop support for opening files on Linux

[timlehr]

Feature Suggestion

Currently you have to open files via the File Open dialog once Raven is launched. It would be great if we could add drag & drop support for OTIO files. The file could be opened whenever someone drags a new OTIO file onto the main window.

[agetroortega]

I'll give this one a go for Dev Days.

[agetroortega]

I have a PR ready for review:
#69

[jminor]

This PR added drag & drop support on macOS, but there is still work needed to bring this feature to Windows and Linux (and maybe WASM also?)

[jminor]

This PR added drag & drop support on Windows, but we'd still like to bring this feature to Linux