Allow tweaking the style onscreen timestamp label

This small patch makes it possible to change the placement, font, font size, font color, and background color of the onscreen timestamp label. However, it does not add GUI configuration or per-project persistence... it's simply "better than nothing"!

Edit the class `TimestampLabelConfig` in `timemanagerguicontrol.py` to change the appearance of the timestamp label globally.

```python
class TimestampLabelConfig(object):
    """Edit configuration for the timestamp label here, in liu of GUI control"""
    font = "Arial"      # Font names or family, comma-separated CSS style
    size = 4            # Relative values between 1-7
    fmt = "yyyy-MM-dd hh:mm:ss.zzz"  # Uses Qt's QDate format, see: http://qt-project.org/doc/qt-4.8/qdate.html#toString
    placement = 'NW'    # Choose from N, NE, E, SE, S, SW, W, NW
    color = 'black'     # Text color as name, rgb(RR,GG,BB), or #XXXXXX
    bgcolor = 'white'   # Background color as name, rgb(RR,GG,BB), or #XXXXXX

```