Allow hiding tasks + UI fixes
Inside the todo page, the user can now hide completed tasks. The default is still to show them.
In addition, we implement some UI fixes:
- We unset QT_AUTO_SCREEN_SCALE_FACTOR, as otherwise the app is not scaled at all on Unix (it is set to "0" automatically in KDE/Plasma, which basically disables Qt::AA_EnableHighDpiScaling.
- By default, we now hide the progress indicator in todo pages. If desired, the user can use the newly added tool bar button to add the progress editor back.
Closes #393 (closed)