Action: SaveScreenshot / CopyScreenshot

[christianparpart]

since the logic is already there (for debugging purposes initially), we can provide an input mapping to let the user create screenshots of the current terminal screen on the fly.

• SaveScreenshot: takes a screenshot and saves it into a file with a given pattern
• CopyScreenshot: takes a screenshot and puts it into the system clipboard, see https://doc.qt.io/qt-5/qclipboard.html#setImage

[Yaraslaut]

Connected to #210