It was only used to support imgur async upload and some dialogs.
In case we need async code in future, we can consider glib's async
facilities.

Closes #122

For now, this requires a considerable amount of work therefore
will be implemented on its own MR.

Only supported in wlroots-based compositors.
There may be leaks!

By very basic I mean an unsupported image will be shown for all
captures modes. But hey, this is better than crashing!

When the save path passed from cli is not a directory it should
not be saved in preferences. I also took the chance to simplify
the code a bit, no need to check the same thing in two places.

Closes #2

screenshooter_take_screenshot used to be blocking but isn't for some
time already therefore we need a callback to asynchronously notify when
the screenshot action was completed or cancelled so the plugin button
can be unblocked, right now it was never blocking because that function
returned immediately.

This also simplifies the code by removing conditionals.