Time representation in class TimePane must be float, not integer

This is causing misbehavior when the video has 30 fps.

By the way, the variable name mfps is used throughout the code. This is a misnomer. Those variables contain actually the value of the duration of a video frame, expressed in milliseconds. This should be changed to a more sensible name, like frame_duration, or whatever.

This issue is related to issue #123 (closed) and the fix for both will eventually be done together.