Frontend: Reorganize files

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
setup.py: cosmetic

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Extract progressive list from mainwindow module

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Frontend: Move CellRendererLabels out of the mainwindow module

Signed-off-by: Jerome Flesch <jflesch@gmail.com>