Implement drag'n'drop for the Mastodon launcher

[maarzt]

Use case

A user has Fiji, the Mastodon launcher and the file browser open on his computer. The user would like to open a mastodon project by dragging a *.mastodon file from the file browser to the Mastodon launcher.

Acceptance criteria

Dragging a *.mastodon file via drag'n'drop from the file browser to the Mastodon laucher opens the mastodon project.

[tinevez]

Normally it is implemented in Mastodon. But if you do this in Fiji, there is a bug in Bioformats that prevents it to work. The error message is weird:

[stefanhahmann]

Resolved by #214