Redesign rename/overwrite dialog

Reworked components in the rename dialog, as discussed in https://discuss.kde.org/t/simplify-the-file-conflict-overwrite-dialog-redesign/5938/51


| Before | After |
| ------ | ------ |
| ![rename_dialog_-_before](/uploads/70a26f4c788e843d1b94c49fc76b27a8/rename_dialog_-_before.png) | ![rename_dialog_after_-_with_space_above_buttons](/uploads/8508de801c454789dc6a6211e37992f8/rename_dialog_after_-_with_space_above_buttons.png) |