download-manager: Prevent unintended automatic opening of saved files with Gecko >= 26
modules/download-manager.js: make check for Downloads.jsm more robust

Just checking for the presence of Downloads.jsm is not enough, as
Firefox 24.5.0esr has the module but is missing the getList function.
Now we check for getList as well.
conkeror.xul: add fullscreenbutton to conkeror window

This adds a "fullscreen" button to the window decoration on platforms that
support it.  (OS X 10.7 Lion and later and Windows)

https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Attribute/fullscreenbutton