redesigned variables tab

names and values listed on the left, input box for the selected variable on the right.
dependencies are highlighted in the list, and there's a list of variables which use the current variable.

ref. #186, in a way.
variable list is sortable
change current variable after deleting; regen off by default
move dependency display to a list beneath current var; auto-regen turned back on
move the delete variable button to the list
move the add variable button to the bottom of the list