title-box: Make EphyTitleBox honor the location window property

When the location bar is disabled in the window chrome, use the title
mode unconditionally and don't allow to switch to entry mode. App mode
disables the location in the window chrome by default so the
EphyTitleBox doesn't need to know the embed mode.

https://bugzilla.gnome.org/show_bug.cgi?id=724170