[patch] consistent file save UI behaviour for overwriting files

The UI for downloading files is split between Dillo (which prompts for
a file  to save to) and  the downloads DPI (which  prompts for another
file if  the first file  already exists).  This leads  to inconsistent
behaviour for  "Save link  as file"  depending on  whether the  URL is
cached or  not - if  it is cached then  any existing file  is silently
overwritten but if it  is not cached we go to  the downloads DPI which
prompts before overwriting anything.

These two patches move the UI logic from the downloads DPI into Dillo.
This puts  all the UI logic  in the same place  and ensures consistent
behaviour for "Save link as file" whether or not the URL is cached.

    http://starurchin.org/dillo/patch/downloads-dpi_file_test_remove.html
    http://starurchin.org/dillo/patch/uicmd-cc_file_test_add.html

Comments?  Shall I push this?

Regards,

Jeremy Henty