Implement sending images directly from clipboard

[maximbaz]

I tend to make screenshots and send them in Signal relatively often, so it would be quite helpful if we could do it in gurk.

This PR adds support for just that - when clipboard contains an image, send it.

I wasn't sure about the interface, I saw it's possible to attach files using file://path/to/file so as a easy idea I made file://clip be treated like a special path, that looks into clipboard instead of the disk.

Feel free to change this to whatever makes more sense!

Some implementation details:

• There is already a library for interacting with clipboard, and it supports getting images out.
• It returns RGBA pixels for the image, but we need to send a file.
• I picked png as an extension, and so I convert the RGBA to PNG and send it as is, along with hardcoded MIME type and file name.

What do you think?

[boxdot]

Thank you. This is great feature. I just have some minor comments.