feat(list): implement GlobalIndex helper (#574)

* feat(list): implement GlobalIndex helper

This commit introduces a new `filteredItem` field called `index` which
stores the index of the filtered item in the unfiltered list. This
allows to get at runtime the unfiltered list index for the selected (and
possibly filtered) item.

This is the only solution I found to use `SetItem` with a filtered
list.

The name `GlobalIndex` might not be ideal, I'm happy to change it to
something else. (`UnfilteredIndex`?)

Fixes: #550

* docs(list): improve Index() documentation

list/list.go

@@ -53,6 +53,7 @@ type ItemDelegate interface {
 }
 
 type filteredItem struct {
+	index   int   // index in the unfiltered list
 	item    Item  // item matched
 	matches []int // rune indices of matched items
 }
@@ -484,12 +485,26 @@ func (m Model) MatchesForItem(index int) []int {
 	return m.filteredItems[index].matches
 }
 
-// Index returns the index of the currently selected item as it appears in the
-// entire slice of items.
+// Index returns the index of the currently selected item as it is stored in the
+// filtered list of items.
+// Using this value with SetItem() might be incorrect, consider using
+// GlobalIndex() instead.
 func (m Model) Index() int {
 	return m.Paginator.Page*m.Paginator.PerPage + m.cursor
 }
 
+// GlobalIndex returns the index of the currently selected item as it is stored
+// in the unfiltered list of items. This value can be used with SetItem().
+func (m Model) GlobalIndex() int {
+	index := m.Index()
+
+	if m.filteredItems == nil || index >= len(m.filteredItems) {
+		return index
+	}
+
+	return m.filteredItems[index].index
+}
+
 // Cursor returns the index of the cursor on the current page.
 func (m Model) Cursor() int {
 	return m.cursor
@@ -1256,6 +1271,7 @@ func filterItems(m Model) tea.Cmd {
 		filterMatches := []filteredItem{}
 		for _, r := range m.Filter(m.FilterInput.Value(), targets) {
 			filterMatches = append(filterMatches, filteredItem{
+				index:   r.Index,
 				item:    items[r.Index],
 				matches: r.MatchedIndexes,
 			})