fix: closing newly loaded sequence

The newly aquired video was closed directly after opening it, resulting in an error when loading a new sequence into an existing project. This probably is some remaining code from older handling of the squences. No freeing is neccessary, all values (min/max frame) are updated and the old file is closed by cv::VideoCapture.

This just resolves the error, but it might still be interesting to look into loading a new sequence. There might be project-parameters which should not be used with the new sequence (e.g. trajectories, which are currently handled specifically or XSens c3d-files). A strucuted solution would be more welcome than a solution of hand-written resetting.

Related #88 (closed)