Make columns sortable and resizable

Minor:

• Made "Current Session"->"Program" and "Application Autostart"->"Program" resizable.
  • Reason: Users might not perceive "window resizing" as possibility to resize columns
  • Note: This does not affect the auto-stretch functionality of the "Program" column, as long is it is not used.

Major:

• Made table in "Application Autostart" sortable
  • Added: Sort interface to XfaeModel (GTK_TYPE_TREE_SORTABLE)
  • Added: Sort function for each column
  • Renamed: xface_model_sort_items() -> xface_item_sort_default()