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()