Load contents from file

[boneskewer69]

I would like to to have an alternative way to get text into Boop. For example draging a file into Boop or opening it with a dialog. The actual file shouldn't get modified. But the context should get appended.

This would allow users to load data into Boop, without needing to open it in another application and copy it over. I frequently encounter this situation and don't want to open an editor just to paste the contents into Boop.

Issue related is #227 if I'm not mistaken.
In a way also #231...