Update RemoveAllFiles dialog

[TheOneRing]

Goal of this change is to prepare the removal of addModalLegacyDialog

Please also have a look on #11564 where a qml is used instead

Old:

[TheOneRing]

@tbsbdr for the design I tried to use the available space better.

[tbsbdr]

I'd remove the dialog at all.
if all files got deleted from the server, the default should be that all files get also deleted from the client. the server is the source of truth; files can be restored from the server trashbin if this was done unintentionally.