Add more options to Test Signal Generator

[kumaakh]

Two New types of signals shoule be added to Test Sginal Generator

1. Frame Index Overlay: shows running numbers in the middle of the screen
2. Timestamp Overlay: shows current timestamp on each frame generated

Following care should be taken here:

1. Make sure that the numbers/timestamps are aligned so that they dont appear dancing
2. Make sure a fixed width font is used.
3. The background colour (default black) and foreground colour (default green) should be configurable
4. The timestamp format (default: hh:mm:ss.nnn) should be configurable.
5. The placement of the text box should be configurable: default to vertical middle of the image and aligned so that the text does not appear to be jumping.
6. The font size should be selected automatically to maximize the benefit of a counter. It should also be possible to override the size

[kumaakh]

@mraduldubey any one available to take this ?