Improving File Management Preferences dialog

[Luca Ferretti <elle uca libero it>]

Description of problem
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The "File Management Preferences" dialog has some issues
     1. It still use the (deprecated) GtkOptionMenu widget
     2. The "Icon Captions" tab don't follow the HIG specs
              * "Label the drop-down list with a text label above it or
                to its left"
              * explanatory text in dialog
     3. Global options are mixed with specific view options, for example
        list view options appears in both "Views" and "List Columns"
        tabs


So I picked up my glade and I tried to build up something. 

Actual results
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Here a detailed list of changes (items starting for '>' means just for
fun, not implementable actually :-)

General
      * dialog spacings
      * renamed tabs labels

Displaying
      * removed hbox11 (containing default_view_optionmenu): HIG said
        "Do not use drop-down lists with fewer than three items".
      * s/sort_order_optionmenu/sort_order_combobox
      * note that sort_order_optionmenu use bad capitalization
      * add a "Manually" sorting option in sort_order_combobox
      * removed the "Tree View Defaults": now we are spatial, we can
        keep it simply as a GConf key, don't expose in UI
      * moved list and icon view specific controls in appropriate tab
      * use radio controls to choose default view
      * > add a detailed (MacOSX Finder style) selection on showed
        volumes on Desktop [1]

Management	
      * add a couple of radio (spatial_radiobutton and
        browser_radiobutton) to connect to
        /apps/nautilus/preferences/always_use_browser GConf key
      * Use the "Behavior" section to set spatial/browser UI. Use the
        "Items Activation" to set click policy.
      * delete the "Executable Text Files" section and pack it as
        dropdown list (scripts_combobox) in "Item Activation" section

Icon View
      * s/iconview_zoom_optionmenu/iconview_zoom_combobox
      * add labels (not so good but HIG compliant) for
        captions_*_optionmenu and convert them to ComboBox

List View
      * s/listview_zoom_optionmenu/listview_zoom_combobox
      * > add a checkbox to enable Classic Finder style tree view [2]

Preview
      * s/optionmenu/combobox 

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For glade file see http://bugzilla.gnome.org/show_bug.cgi?id=143444

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[1] we have already the /apps/nautilus/desktop/volumes_visible GConf
key, but it's a TRUE/FALSE for ALL volumes (hard, removable, server). 

[2] if and when someone will implement it, IMHO we don't need to add it
as a third view.