Allow taking screenshots using a delay

[py-crash]

For a bug discussion about the cosmic alpha I wanted to take a screenshot of the launcher when accessing it using the "Alt+Tab" key combination, however this is impossible with the stock screenshot tool (cosmic-screenshot).

For it a delay from a couple of seconds should be needed. For example, select the rectangular region (or the whole screen), set a 3 second delay,press the capture button and then press the Alt+Tab combination.