bumped version number to 0.4.3
Merge branch 'release-0.4.3' into dev
o feature54 : DownloadDemoDipydocs class : download and remove demonstration
              DipyDocs.
o feature53 : Splashscreen is now centered with the main window.
o feature52 : Fixed a bug in the way the UI updated the commentary zone : this
              zone can now be updated only if a specific flag (bool
              UI::selected_text_and_blocked_commentaries) is set to false. This
              flag is set to true if, in RMODE, a portion of text has been
              selected.
o feature51 : The PosInTextRanges class doesn't use an unordered_map anymore but
              a map, allowing to store sorted keys.
o feature50 : (MainWindow::add_open_menu) added a special display to
              "File>Open>" if no Dipydoc could be found.