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.

runtime/help/keybindings.md

@@ -492,7 +492,7 @@ conventions for text editing defaults.
     "Ctrl-z":         "Undo",
     "Ctrl-y":         "Redo",
     "Ctrl-c":         "Copy|CopyLine",
-    "Ctrl-x":         "Cut",
+    "Ctrl-x":         "Cut|CutLine",
     "Ctrl-k":         "CutLine",
     "Ctrl-d":         "DuplicateLine",
     "Ctrl-v":         "Paste",
@@ -616,7 +616,7 @@ are given below:
         "Ctrl-z":         "Undo",
         "Ctrl-y":         "Redo",
         "Ctrl-c":         "Copy|CopyLine",
-        "Ctrl-x":         "Cut",
+        "Ctrl-x":         "Cut|CutLine",
         "Ctrl-k":         "CutLine",
         "Ctrl-v":         "Paste",
         "Home":           "StartOfTextToggle",