Fix for Issue 46: Added a sorting menu for networks, stations and channels

By default, the inventory tree is now sorted in ascending order for networks, stations and channels. When the user right clicks on the inventory tree, a menu will popup allowing them to sort the selection type in ascending/descending order. I.e., if they right click on a network, the menu will give them the option to sort networks in ascending/descending order. If they right click on a station, the menu will let them sort stations in ascending/descending order, and the same is true of channels. The message text at the bottom left of the window will also update to display "Sorting..." while the process is happening, and then it will say "Sorting... Done." when it is complete.