CustomFileSystemModel: simplify logic in Checked
CustomFileSystemModel: simplify logic in Checked

We can simply this because of setDataInternal(), which does all
modifications and sends a single emit() at the end (instead of
sending an emit() every time we change anything).

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui