Add optional "Save As" default file format

Add a new "Canvas.SaveAsDefaultType" option in the rootrc file, allowing the user
to specify the default format to be used in the TCanvas' "Save As" file dialog,
as requested on the forum (http://root.cern.ch/phpBB3/viewtopic.php?f=3&t=18090)
Merge branch 'v5-34-00-patches' of https://root.cern.ch/git/root into v5-34-00-patches