Implement open/select/save file dialog.

[marco-sacchi]

Implement a dialog for opening / selecting and saving a file.

The window must contain the following controls:

• editbox for entering the file name or wildcard to filter the displayed files
• listbox to view files in the current folder
• listbox for viewing the drives and folders of the current folder
• open, save and cancel buttons

The difference in behavior between opening and saving consists in checking that the file exists when pressing the open / save button.

When moving in the file list or when changing directories the editbox must be synchronized accordingly, displaying the name of the selected file.

When in save mode, display a message box to confirm overwriting if the file already exists.

[marco-sacchi]

It remains to implement the state recovery code after a critical error (see #28).