main: rewrite (save as) screenshot saving

This picks a default directory and file name. If on Windows and save-as screenshot saving is enabled, it asks the user, first defaulting to the default screenshot path, and with a default filename in the format `[title_id]_[year-mt-dy_hr-mn-sc-msc].png`. Otherwise, or on Linux for now, it simply saves a file in that directory with that file name.