Creating a FileChooser to select the target folder

Adding a button on the operations layout to open a File Chooser to select
the target folder where to do the replacement. Also automatically fetches
the chosen folder as soon as confirmed.
Creating option to rename folders instead of files

Checkbox has been added to the MainWindow so that one can choose to rename
folders instead of files. When checked/unchecked the list of files (or
folders) will be automatically fetched as well. Had to modify the Renamer
adding a boolean to say if we are renaming directories or files and modify
the file filter used (it could not be static anymore).