Add temporary prefs checkbox to use internal PDF reader

And use when opening PDFs in My Library from items list and
zotero://open-pdf

This also adds "Zotero" to the main PDF reader pref, which will be the
real way to set this, but that option is hidden for now so that group
library items will still open with the configured reader.

components/zotero-protocol-handler.js

@@ -1182,7 +1182,7 @@ function ZoteroProtocolHandler() {
 			var opened = false;
 			if (page) {
 				try {
-					opened = await Zotero.OpenPDF.openToPage(path, page);
+					opened = await Zotero.OpenPDF.openToPage(item, page);
 				}
 				catch (e) {
 					Zotero.logError(e);