Backport PR #10589 on branch 5.x

Merge pull request  #10589 from Carreau/54whatsnew

Synchronize and expand what's new on 5.4

docs/source/whatsnew/version5.rst

@@ -5,6 +5,14 b''
 IPython 5.4
 ===========
 
+IPython 5.4-LTS is the first release of IPython after the release of the 6.x
+series which is Python 3 only. It backports most of the new exposed API
+additions made in IPython 6.0 and 6.1 and avoid having to write conditional
+logics depending of the version of IPython.
+
+Please upgrade to pip 9 or greater before upgrading IPython. 
+Failing to do so on Python 2 may lead to a broken IPython install.
+
 Configurable TerminalInteractiveShell
 -------------------------------------
 
@@ -16,7 +24,40 b' that allow to customize the class used to start the terminal frontend. This'
 should allow user to use custom interfaces, like reviving the former readline
 interface which is now a separate package not maintained by the core team.
 
+Define ``_repr_mimebundle_``
+----------------------------
+
+Object can now define `_repr_mimebundle_` in place of multiple `_repr_*_`
+methods and return a full mimebundle. This greatly simplify many implementation
+and allow to publish custom mimetypes (like geojson, plotly, dataframes....).
+See the ``Cutom Display Logic`` example notebook for more informations.
+
+Execution Heuristics
+--------------------
+
+The heuristic for execution in the command line interface is now more biased
+toward executing for single statement. While in IPython 4.x and before a single
+line would be executed when enter is pressed, IPython 5.x would insert a new
+line. For single line statement this is not true anymore and if a single line is
+valid Python, IPython will execute it regardless of the cursor position. Use
+:kbd:`Ctrl-O` to insert a new line. :ghpull:`10489`
+
+
+Implement Display IDs
+---------------------
+
+Implement display id and ability to update a given display. This should greatly
+simplify a lot of code by removing the need for widgets and allow other frontend
+to implement things like progress-bars.  See :ghpull:`10048`
+
+Miscs
+-----
 
+* ``_mp_main_`` is not reloaded which fixes issues with multiprocessing.
+  :ghpull:`10523`
+* Use user colorscheme in Pdb as well :ghpull:`10479`
+* Faster shutdown. :ghpull:`10408` 
+* Fix a crash in reverse search. :ghpull:`10371`
 * added ``Completer.backslash_combining_completions`` boolean option to
   deactivate backslash-tab completion that may conflict with windows path.