Add changes file for Qt 5.12.3

+ b7c27591 Print warning when QGeoFileTileCache::init fails creating the directory
+ 0f544f67 Fix incorrect indexing of Mapbox tiles in the cache
+ 2bf874e5 Document QGeoPath::length behavior
+ 6e02f895 Bump version
+ 0af21b1f Fix Map destructor incorrectly deleting MapItemViews
+ 5c3e3464

 Display a notice when requesting a SSL map type without SSL support

Change-Id: Ic5ca192808986433257de28261539801f65fc0ae
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

dist/changes-5.12.3

+Qt 5.12.3 is a bug-fix release. It maintains both forward and backward
+compatibility (source and binary) with Qt 5.12.0 through 5.12.2.
+
+For more details, refer to the online documentation included in this
+distribution. The documentation is also available online:
+
+https://doc.qt.io/qt-5/index.html
+
+The Qt version 5.12 series is binary compatible with the 5.11.x series.
+Applications compiled for 5.11 will continue to run with 5.12.
+
+Some of the changes listed in this file include issue tracking numbers
+corresponding to tasks in the Qt Bug Tracker:
+
+https://bugreports.qt.io/
+
+Each of these identifiers can be entered in the bug tracker to obtain more
+information about a particular change.
+
+****************************************************************************
+*                              QtPositioning                               *
+****************************************************************************
+
+ - Documented QGeoPath::length behavior properly.
+
+****************************************************************************
+*                               UNSPECIFIED                                *
+****************************************************************************
+
+ -  Fixed incorrect indexing of Mapbox tiles in the cache.
+ -  Fixed crash when destroying Maps containing MapItemViews.
+ -  Added a notice to the MapViewer example to inform when a HTTPS-based map
+    is selected without SSL support.