Fix inconsistencies in the preference names used by PDFViewerApplication

Since `preferenceSidebarViewOnLoad` is an enumeration value, it seems better to initialize it with the default one instead of a boolean.
`preferencesPdfBugEnabled` uses a superfluous "s", which is a typo I made when I introduced it.
Merge pull request #5369 from Snuffleupagus/viewer-temp-pref-names

Fix inconsistencies in the preference names used by PDFViewerApplication