Return 0 if the tab "name" was not found
Fix Jira issue #ROOT-6189

In TRootCanvas::SaveAs, change the default file type from ps to pdf
In TGFileDialog, when saving, set a default file name and its extension (if a specific one is selected)
and highlight the name so when the user types something, only the file name is changed
Thanks Axel for the suggestion and the useful feedback