stbt.Display: Add clock on each frame's left top corner

While recording a video with this commit there will be visible clock
on left top corner. Clock displays time when given frame was
processed. In addition when calling a function 'press' we will draw not
only an information that button was pressed but as well time when it was
pressed. This should provide accurate information that will let us find
relation between actions registered in logs and what happened on screen.
stbt.py: Change video text font to HERSHEY_DUPLEX

I'm convinced that this font looks a lot better. At least it doesn't
look like a headline in a traditional newspaper.