Remove Zotero.Browser and add HiddenBrowser.jsm. Post-Fission, web/file
content loads in a separate process, so it's not possible (as best as I
can tell) to directly access the contents of a hidden browser -- it just
appears as about:blank in the parent process. We now use Mozilla's
JSWindowActor mechanism [1] to get page data, including character set
and body text for full-text indexing. We'll have to evaluate other uses
of hidden browsers to see how to handle them.
This also adds include.jsm for loading the Zotero object into a JSM.
[1] https://firefox-source-docs.mozilla.org/dom/ipc/jsactors.html
