browser-plugin: Set windowless mode and don't claim to need XEmbed

NPAPI plugins are windowed by default, so we need to set
NPPVpluginWindowBool value to FALSE on startup. This way the browser
will not create a GtkSocket for a GtkPlug that we are not going to
create. It doesn't make sense to claim that we need XEmbed either.

https://bugzilla.gnome.org/show_bug.cgi?id=757940