moved AboutDialogUI to eotimeseriesviewer/main.py and deleted eotimeseriesviewer/wigets.py

modified documentation:
- changelog.rst for comprehensive listing,
- changelog_visual.rst for visual comparision
- changes with external link to issues visible in AboutDialogUI and sphinx documentation

Signed-off-by: benjamin.jakimow <benjamin.jakimow@geo.hu-berlin.de>