Improve the main window in mobile devices

This small MR improves slightly the use of baobab in mobile devices. In particular, it can show a start window that looks fine in a Pinephone device. However the results window is not fine, since it is too wide. To fully make adaptive as requested in #118 the results window should probably be changed to show only the directory tree when the window is too small with buttons to show the ring chart and treemap chart.