Add SetSelection function for list contexts and use it in most places

The only time we should call SetSelectedLineIdx is when we are happy for a
select range to be retained which means things like moving the selected line
index to top top/bottom or up/down a page as the user navigates.

But in every other case we should now call SetSelection because that will
set the selected index and cancel the range which is almost always what we
want.

pkg/gui/context/traits/list_cursor.go

@@ -45,10 +45,25 @@ func (self *ListCursor) GetSelectedLineIdx() int {
 	return self.selectedIdx
 }
 
+// Sets the selected line index. Note, you probably don't want to use this directly,
+// because it doesn't affect the range select mode or range start index. You should only
+// use this for navigation situations where e.g. the user wants to jump to the top of
+// a list while in range select mode so that the selection ends up being between
+// the top of the list and the previous selection
 func (self *ListCursor) SetSelectedLineIdx(value int) {
 	self.selectedIdx = self.clampValue(value)
 }
 
+// Sets the selected index and cancels the range. You almost always want to use
+// this instead of SetSelectedLineIdx. For example, if you want to jump the cursor
+// to the top of a list after checking out a branch, you should use this method,
+// or you may end up with a large range selection from the previous cursor position
+// to the top of the list.
+func (self *ListCursor) SetSelection(value int) {
+	self.selectedIdx = self.clampValue(value)
+	self.CancelRangeSelect()
+}
+
 func (self *ListCursor) clampValue(value int) int {
 	clampedValue := -1
 	if self.list.Len() > 0 {