The current "Select Linked" operator works based on mouse position and
makes no sense to call from the menus and was removed in rBdd9dfadaac9b.
This patch adds an operator independent from mouse position that just
selects all keys to a corresponding point (and adds back menu entries).
The original operator is renamed to 'select_linked_pick' internally
(this is now more in line to how "Select Linked" works for meshes,