Add ability to save screenshot

I understand the workflow of just using Share, but sometimes you just want to _save_ a screenshot. E.g. if you want to later upload it to a web-based forum that you cannot "share" to.

Please can you add a save feature, either as a separate button next to share, or within the Share list I suppose, although I would prefer a separate button.

Many thanks for creating this app.

Start by tracing the app's existing screenshot creation, cropping, and Share workflow to find where a save action could fit. Clarify the preferred separate button versus Share-list placement and the save destination; done means a created screenshot can be saved for later upload as well as shared.

Written by the indexing model from the issue text.