desktop-entry: separate icon and app_id icon lookup

desktop-entry: separate icon and app_id icon lookup

This patch splits desktop_entry_icon_lookup() into two separate functions
- desktop_entry_load_icon(): load a icon from the configured icon theme
- desktop_entry_load_icon_from_app_id(): load a icon name from a .desktop
  file based on the given app_id and supply it to _load_icon().

The _load_icon() function will be used in a future menu icon
implementation whereas the _load_icon_from_app_id() function is used
within the SSD titlebar window icon lookup routine.