Draw clock & button-press timestamps on output video

Previously it used only hardcoded and non-configurable values. With this commit we can specify in config file:
 - text that is printed after stbt.press will last for float number of seconds (previously hardcoded 3 seconds)
 - we can choose to have timestamp of when key press was received for function stbt.press
 - we can choose to not display any text after stbt.press
 - we can choose to display clock in right top corner