Use a File-save-dialog for path+filename, instead of textbox for name, and folder selector for path.

Other information:
Currently when gnome-screenshot (PrintScreen) is used a default filename is given in an editable textbox and a default directory is given in a combobox.
To save in other direcotory (except favourite) "browse" must be used. It will open the file chooser dialog, set to only accept directories (folder-select). The filename can not be set from here, only the directory. This also has the side effect of me not seeing what existing images there are, and not being able to reuse a name and onlly modify it partially. I have at least 10 times mistakedly created a new directory with the name i was going to give the file and dismissed the dialog. Then returned to gnome-screenshot and realized it was just the directory I was supposed to select. So I end up with a folder named something.png, and then have to reopen the browse dialog and move to parent direcotory. Close, and enter the name in the file-name box. But now the name is occupied by the unwanted directory(!). Aaarg. Have to open Nautilus and remove the directory I accidentally created (since removal is not possible from File chooser dialog...).
Summary:
I can't think of any other (gnome) program that uses the combination of
* folder-selection dialog
and 
* filename (basename, without path) in plain text box.

The reasonable thing to do is of course to use a save-file dialog. It can of course be set up with the same default name and directory as now, but when you need to change it, the user interface would be much easiear.

Perhaps the reasons for current design is something about common to place all screenshots in one directory, but using a save-file dialog with same default would be as easy if default name is OK. Two clicks (Browse and OK) more if you want to change filename (or path). Current design allows filename change without extra clics, but need three clicks (+browsing) to change path.