Make more settings features work

Made a couple more settings features work for the diff tools
Loads more changes! Woo!

Lots of work done, sorta just works for settings.
Add double click edit on settings

Allow double click to edit the currently selected diff tool
when viewing the settings menu
Refactor button visibility

Refactor the visibility of the Set Default, Edit and Remove
buttons based on the number of selected items
Add conditions to remove diff tool

Add conditions to prevent removing a default diff tool
unless it is the only remaining tool.