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Fix #727525 reported by Anders Jonsson

These are all of the same types that the ones pointed out by Christian, just plenty others.

Apply patch provided by Christian himself.
Fix #640216.

Targeting selection, background or toolbar is only relevant for actions.
Folders, to be compared to Nautilus current_folder, is only relevant in profiles.

This is needed because creating automagically a root menu for displaying About item
modifies the menu layout in a unfriendly way.

When the user checks the 'keep this last import mode' box, then the last choice is setup
as a new import mode preference.

Let the user decide what to do if UUID of imported actions already exist.

Renamed preference keys to iprefs-add-about-item and iprefs-alphabetical-order.
Add new iprefs-level-zero key.

Some GConf preference keys were misnamed since 1.11.1 :
- iconditions-command-chooser,
- iconditions-legend-dialog,
- iconditions-folder-uri.
These are now migrated. Unfortunately, the migration code
will have to stay inline for some time.

For now, only drag to external applications (file-manager, text editor, ...) is implemented.
A new NAActionMenu class is defined, derived from new NAObjectItem class.
Preferences 'Add About item' and 'Display in alphabetical order'.

Add schemas for Preferences dialog size and position, and for
DisplayAsSubmenu option