Reorganize Cut and CutLine actions

Change behavior of the Cut action: don't implicitly call CutLine if
there is no selection. Instead, make it return false in this case
and change the default Ctrl-x binding to Cut|CutLine, to make it clear,
explicit and in line with Copy and CopyLine actions.

internal/action/actions.go

@@ -1230,7 +1230,7 @@ func (h *BufPane) Cut() bool {
 		h.Relocate()
 		return true
 	}
-	return h.CutLine()
+	return false
 }
 
 // DuplicateLine duplicates the current line or selection