Select multiple items

[mattstermiller]

Allow the user to select multiple items for moving, copying, deleting, renaming (prepend, append to name, append to extension).

Allow multi-selection by:

• <space> to toggle selection of the item under the cursor
• <s-space> to toggle all items from the last selected item up to and including the item under the cursor (if there was no other selected item, just select the cursor item)
• <c-a> to select all
• <Esc> to clear selection

When there are any items multi-selected:

• The status bar should show the count and total size of the selections

Implementation details:

• Replace SelectedItem with CursorItem (single item) and SelectedItems (list)
• Add computed ActionItems (list) that is SelectedItems with fallback to CursorItem