DOC: add whats_new for qt configuration editor.

doc/users/whats_new/2016-04_qt_config-AL.rst

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+Improvements for the Qt figure options editor
+---------------------------------------------
+
+Various usability improvements were implemented for the Qt figure options
+editor, among which:
+- Line style entries are now sorted without duplicates.
+- The colormap and normalization limits can now be set for images.
+- Line edits for floating values now display only as many digits as necessary
+  to avoid precision loss.  An important bug was also fixed regarding input
+  validation using Qt5 and a locale where the decimal separator is ",".
+- The axes selector now uses shorter, more user-friendly names for axes, and
+  does not crash if there are no axes.
+- Line and image entries using the default labels ("_lineX", "_imageX") are now
+  sorted numerically even when there are more than 10 entries.