Disable horizontal autoscrolling in QTreeView

Part of #168 .

If the contents of the QTreeView widget (the widget on the left that shows files within the directory structure) are wider than what fits into its current size, the widget will scroll all the way to the left whenever an item is (double) left-clicked, even when this actually hides (part of) the text of the item. This is probably because it considers the items to run the full width of the widget (as shown by the gray bar that appears when an item is selected), even when the item is several layers deep.

This MR disables this automatic scrolling, by remembering the scroll bar position before scrolling and setting the bar back to that value after scrolling. It's a bit hacky, but the only solution I could find without trying to rewrite all of scrollTo.