Restore file download UI when OnlyOffice plugin is disabled

[gvengel]

Previously, the OnlyOffice plugin statically registered mime types with Hooks.Register.XDGOpen on initialization; however, this meant that if you disabled the plugin, those mime types would still remain associated with the plugin. In that case, any office document would return a blank page, since it would still attempt to open with the disabled OnlyOffice plugin handlers.

This change adds a new Hooks.Register.XDGOpenFunc hook, so that code in xdg-open.js can be evaluated dynamically. With it, mime type overrides can be returned only when the OnlyOffice plugin is enabled.

While the private API for xdg_open now accepts an array of functions returning a string (similar to the CSS hook), the public API for Hooks.Register.XDGOpen and Hooks.Get.XDGOpen remains unchanged, so this shouldn't break any 3rd party plugins using it.

Fixes issue #717.

[gvengel]

Note, this may be tangentially related to issue #715, as my motivation for disabling OnlyOffice is that I've found it corrupts some documents on save, and that PR is another possible mitigation.

[mickael-kerjean]

You're right, it will take me a bit of time to test this out but I'll keep you up to date