Rework -a Option

[Cyanistic]

Reworks the -a option to show a drag all files button instead of always dragging all files together. (#33)

[nik012003]

Thank you so much! It looks good, there's only one small issue:
-at or -atk are broken since we do setup_drop_target() when we create ListWidget , and it seems like appending list.widget on the outer_box doesn't make the drop target work for some reason.
I tried debugging it quickly and found out that my assumptions were correct and that putting something like

    let outer_box = outer_box.upcast::<Widget>();

    setup_drop_target(&list.list_model, &outer_box);

at the end of the create_outer_box() function works. (but you'd need to change the function return type to Widget instead of Box. I guess the best way would be to call setup_drop_target() directly in the main when we build the UI.

EDIT: alright, should be fixed in my commit, thank you!